Nanostructure and nanocomposite based compositions and photovoltaic devices

ABSTRACT

Nanocomposite photovoltaic devices are provided that generally include semiconductor nanocrystals as at least a portion of a photoactive layer. Photovoltaic devices and other layered devices that comprise core-shell nanostructures and/or two populations of nanostructures, where the nanostructures are not necessarily part of a nanocomposite, are also features of the invention. Varied architectures for such devices are also provided including flexible and rigid architectures, planar and non-planar architectures and the like, as are systems incorporating such devices, and methods and systems for fabricating such devices. Compositions comprising two populations of nanostructures of different materials are also a feature of the invention.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/656,802, filed Sep. 4, 2003, which claims priority to and benefit ofU.S. Provisional Patent Application No. 60/408,722, filed Sep. 5, 2002,“NANOCOMPOSITES” Mihai Buretea et al., U.S. Provisional PatentApplication No. 60/421,353, filed Oct. 25, 2002, “NANOCOMPOSITE BASEDPHOTOVOLTAIC DEVICES” Erik Scher et al., and U.S. Provisional PatentApplication No. 60/452,038, filed Mar. 4, 2003, “NANOCOMPOSITE BASEDPHOTOVOLTAIC DEVICES” Erik Scher et al., each of which is incorporatedherein by reference in its entirety for all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

Portions of this invention may have been made with United StatesGovernment support under National Reconnaissance Office grantNRO-03-C-0042. As such, the United States Government may have certainrights in the invention.

FIELD OF THE INVENTION

The present invention is in the field of nanostructures. Moreparticularly, the invention provides devices, such as photovoltaicdevices, comprising nanostructures, which optionally are part of ananocomposite. Compositions comprising two populations of nanostructuresof different materials are also features of the invention, as aremethods of and systems for producing photovoltaic and other (e.g.,layered) devices.

BACKGROUND OF THE INVENTION

Solar energy has long been looked to as a potential solution to the everincreasing energy needs of the planet's population. Increasing costs ofmining fossil fuels, increased concerns over “greenhouse” emissions, andincreasing instabilities in regions that house large reserves of fossilfuels have furthered interest in exploiting alternative energystrategies, including solar energy sources.

To date, solar energy conversion has generally relied upon either thedirect harvesting of solar thermal energy, e.g., in heatingapplications, or in thermoelectric conversion, or through the directconversion of photonic energy to electrical energy through the use ofphotovoltaic cells.

Current photovoltaic devices or cells employ thin layers ofsemiconductor material, e.g., crystalline silicon, gallium arsenide, orthe like, incorporating a p-n junction to convert solar energy to directcurrent. While these devices are useful in certain applications, theirefficiency has been somewhat limited, yielding conversion efficiencies,e.g. solar power to electrical power, of typically marginally betterthan 10%. While efficiencies of these devices have been improvingthrough costly improvements to device structure, it is believed thatphysical limitations on these devices mean they will, at best, achieve amaximum efficiency of around 30%. For ordinary energy requirements,e.g., public consumption, the relative inefficiency of these devices,combined with their relatively high cost, when compared to other meansof energy generation, have combined to inhibit the widespread adoptionof solar electricity in the consumer markets. Instead, such systems havebeen primarily used where conventionally generated electricity isunavailable, e.g., in remote locations, terrestrial or otherwise, orwhere costs associated with bringing conventionally generatedelectricity, to a location where it is needed, more closely match thecosts of photovoltaic systems.

Because of their construction and efficiency, currently marketedphotovoltaics also come with a number of physical requirements. Forexample, because of their relative inefficiency, as well as their rigidconstruction, photovoltaic systems typically require adequate flat spacethat has appropriate sun exposure at all times, or at least during peaktimes, to meet the requirements for electricity for which the system isused.

Despite the issues with current photovoltaic technology, there is stilla desire and a need to expand usage of solar electricity. In particular,there is generally a need for an improved photovoltaic cell that has oneor more of: increased energy conversion efficiency, decreasedmanufacturing costs, greater flexibility and/or reasonable durabilityand/or longevity. The present invention meets these and a variety ofother needs.

SUMMARY OF THE INVENTION

The invention includes nanocomposite based photovoltaic devices andtheir constituent elements, systems that incorporate such devices andelements, and methods of and systems for fabricating such devices andelements. For example, photovoltaic devices that include alignednanostructures (e.g., nanocrystals) in a photoactive layer are an aspectof the invention. In a related aspect, devices comprising one or moretypes of nanostructures, including photovoltaic devices comprisingmultiple types of such structures, whether aligned or not, are anotheraspect of the invention. In either case, these devices can include oromit non-nanostructure elements in an active layer. For example, thenanostructures of such an active layer can be fused, partially fused orsintered to provide electron and/or hole carrying properties, e.g.,between electrodes. Polymers, e.g., conductive polymers, can be combinedwith nanostructure elements in a photoactive layer, though such polymersare not required in many embodiments herein. Systems that comprise suchdevices, as well as methods and systems for fabricating such devices,are also an aspect of the invention. Active layer compositions, e.g.,that can be used in photovoltaic and other devices, as well as relatedmethods and systems, are an additional feature of the invention.

Accordingly, in a first general set of photovoltaic device embodiments,the invention provides photovoltaic devices. The devices include a firstelectrode layer and a second electrode layer. A first photoactive layeris disposed between the first and second electrode layers. Thephotoactive layer is disposed in at least partial electrical contactwith the first electrode along a first plane, and in at least partialelectrical contact with the second electrode along a second plane. Thephotoactive layer comprises material that exhibits a type II band offsetenergy profile, and includes a first population of nanostructures (e.g.,nanocrystals) each having at least one elongated section orientedpredominantly normal to at least the first plane.

The nanostructures can include any of a variety of structures, e.g.,branched nanocrystals having more than one elongated segment, e.g., fourelongated segments connected at a common apex, and arranged in asubstantially tetrahedral geometry (e.g., nanotetrapods). Thenanostructures can include materials that facilitate function in thephotoactive layer, e.g., optionally including at least a portion that iscomprised of a semiconductor selected from Group II-VI, Group III-V orGroup IV semiconductors or alloys thereof. For example, the populationof nanostructures can include nanocrystals that include one or more of:CdS, CdSe, CdTe, InP, InAs, ZnS, ZnSe, ZnTe, HgTe, GaN, GaP, GaAs, GaSb,InSb, Si, Ge, AlAs, AlSb, PbSe, PbS, or PbTe. The precise form of thenanostructure can vary, e.g., optionally including any of: ananocrystal, a nanowire, a single-crystal nanostructure, adouble-crystal nanostructure, a polycrystalline nanostructure, and/or anamorphous nanostructure. The population of nanostructures can includestructures that include more than one subcomponent, e.g., such asnanocrystals that comprise a core of a first semiconductor material anda shell of a second (different) semiconductor material. For photovoltaicdevices, the first and second semiconductor materials typically displaya type-II band offset profile. Any of a variety of materials can be usedto achieve this desirable property, including nanocrystals in which thecore comprises CdSe and the shell comprises CdTe, or nanocrystals inwhich the core comprises InP and the shell comprises GaAs. Thesenanostructure features are generally applicable to the other device,method, composition and system embodiments noted below.

The photoactive layer optionally comprises a matrix, e.g., in which thenanocrystals are suspended or disposed. For example, the layer caninclude nanocrystals disposed in a conductive polymer matrix. Thenanocrystals are optionally coupled to the polymer matrix, e.g., via acovalent chemical linkage. For example, in one embodiment, the chemicallinkage comprises a ligand coupled at a first position to an outersurface of the nanocrystal and at a second position to the polymermatrix. The nanocrystals are optionally electrically coupled to thepolymer matrix. Either the nanocrystals or the matrix (or both) canconduct holes or electrons. Furthermore, as noted in more detail herein,the matrix is optionally omitted from the devices herein, withnanostructures providing both electron and hole carrying properties tothe electrodes (e.g., where two or more nanocrystal types are fused,partially fused or sintered together, optionally in the absence of anyadditional conductive (or other) polymer in the matrix).

The arrangement of nanostructures in the photoactive layer can bemodified to enhance the activity of the photoactive layer. For example,in one aspect, one or more of the nanostructures or nanostructure typesin the layer are positioned predominantly closer to the first electrodethan to the second electrode (e.g., to facilitate electron or holeconductance to the relevant electrode). Similarly, the layer can includecomponents that similarly facilitate properties of the layer, e.g., thelayer can include a hole or electron blocking layer disposed between thephotoactive layer and the first or second electrode. Similarly, thelayer optionally includes a hole blocking layer disposed between thephotoactive layer and the first electrode and an electron blocking layerdisposed between the photoactive layer and the second electrode (e.g.,to control flow of electrons or holes to the relevant electrode).

The nanostructures can include multiple distinct subtypes. For example,the population of nanostructures in the photoactive layer can include atleast two different nanocrystal subpopulations, each nanocrystalsubpopulation, e.g., having a different absorption spectrum. Thus, thedifferent nanocrystal subpopulations optionally include differentcompositions, different size distributions, different shapes and/or thelike.

In one aspect, the nanostructures in the photoactive layer collectivelycomprise at least two inorganic materials. For example, thenanostructures optionally comprise a core of a first inorganic materialand a shell of a second inorganic material, and/or the photoactive layercan comprise at least two types of nanocrystals. Any of thenanostructures in the photoactive layer can optionally be fused,partially fused, and/or sintered. Where the nanostructures are fused,partially fused, and/or sintered, e.g., the cores of at least twoadjacent nanostructures in the photoactive layer are optionally in atleast partial electrical contact, and the shells of the at least twoadjacent nanostructures, or at least two additional nanostructures, areoptionally in at least partial direct electrical contact.

The photoactive layer optionally includes one or more sublayers. Forexample, the photoactive layer can include at least two activesublayers, with each of the active sublayers comprising a plurality ofnanocrystals of at least one nanocrystal type. In one such embodiment,at least one of the at least two sublayers comprises an n-type sublayerand at least one of the two sublayers comprises a p-type sublayer,optionally meeting at one or more junction in the photoactive layer(e.g., a p-n or n-p-n or other junction). In an alternate relatedembodiment, the photoactive layer includes at least one sublayer thatincludes a blend of p and n nanocrystals.

In addition to the photoactive layer comprising sublayers, the overalldevice architecture can also be layered. For example, the device caninclude a plurality of photoactive layers (at least a second photoactivelayer, and, optionally, more than two such layers). One or moreelectrode (and, typically, two electrodes) can be placed in electricalcontact with any such photoactive layer of the device. For example, thedevice can include a third electrode layer, a fourth electrode layer anda second photoactive layer disposed between the third and fourthelectrode layers. In this embodiment, the second photoactive layer isoptionally disposed in at least partial electrical contact with thethird electrode along a third plane, and in at least partial electricalcontact with the fourth electrode along a fourth plane. Optionally, thesecond photoactive layer exhibits a type II band offset energy profile,and comprises a second population of nanostructures each having at leastone elongated section oriented predominantly normal to at least thethird plane, and having a different absorption spectrum from the firstpopulation of nanostructures. The third electrode layer, fourthelectrode layer and second photoactive layer are optionally attached to,but electrically insulated from, the first electrode layer, secondelectrode layer and first photoactive layer.

Similar to the elements of the photoactive layer, the electrodes canalso be selected to regulate overall device properties. For example, theelectrodes can be made of any suitable conductive material, selectedbased upon charge carrying capabilities, environmental toleranceproperties, or the like. For example, in one aspect, at least one of theelectrodes optionally comprises aluminum or another metal. Furthermore,the properties of the electrodes or photoactive layers can be selectedto provide a desired overall device property. For example, where aflexible photovoltaic device is desirable, the first and/or secondelectrodes or the photoactive layer can be selected to be flexible.Similarly, the first and/or second electrodes and/or photoactive layeroptionally include additional device elements to protect the device fromits working environment, and/or to enhance device properties. Forexample, in one aspect, any of the electrodes or the photoactive layer(or any combination thereof), can include a transparent layer (e.g., atransparent conductive layer). For example, in one aspect, the deviceincludes a transparent support layer at least partially covering thefirst or second electrode, or at least partially covering thephotoactive layer, or at least partially covering a combination thereof.Similarly, the device can include one or more sealing layers, e.g., thephotoactive layer and/or one or more of the electrodes can comprise orbe sealed within a sealing layer. For example, in one embodiment, thephotoactive layer is hermetically sealed. In one example embodiment, thedevice comprises at least first and second sealing layers, the first andsecond sealing layers, the photoactive layer and first and secondelectrodes being sandwiched between the first and second sealing layers.

The overall architecture of the device can be selected based upon theuse to which the device is to be put. For example, the overall devicecan comprise a planar or a non-planar architecture. For example, thedevice optionally comprises a convex architecture to enhance efficiencyof the device in settings where convex architecture is desired.Similarly, in one embodiment, the first electrode layer, the photoactivelayer and the second electrode layer are optionally oriented in a coiledarchitecture, or in a reciprocating stacked architecture.

In a related second set of embodiments, the nanocrystals are optionallyoriented as noted in the embodiments above, but can, optionally, beoriented differently. That is, the nanostructures can be in any randomor non-random orientation with respect to the various planes andelectrodes noted above. In these embodiments, the nanostructures includeat least one of the inorganic materials of the photovoltaic device, andoptionally two or more such materials. The polymer components of thematrix of the photovoltaic layer noted above is an optional feature ofthese embodiments (the polymer can be retained or omitted in thephotoactive layer).

Thus, in this second set of embodiments, a photovoltaic device isprovided. The device includes a first electrode layer and a secondelectrode layer. A first photoactive layer is disposed between the firstand second electrode layers. The photoactive layer is disposed in atleast partial electrical contact with the first electrode along a firstplane and in at least partial electrical contact with the secondelectrode along a second plane, wherein the photoactive layer comprisesa first inorganic material and a second inorganic material differentfrom the first inorganic material. In PV devices, the first and secondinorganic materials typically exhibit a type II band offset energyprofile. The photoactive layer includes a first population ofnanostructures, which nanostructures comprise the first inorganicmaterial, the second inorganic material, or a combination thereof.

All of the features noted above with respect to nanostructure type,shape and composition, electrode composition and configuration anddevice architecture apply to these embodiments as well. Any of theoptional transparent/ blocking/ sealing layer embodiments of the firstset of embodiments can be features of the second set of embodiments aswell. Similarly, device architecture features noted above are applicablehere as well. Unless noted otherwise, any feature of the first class ofphotovoltaic device embodiments noted above or otherwise herein can beapplied to this second class of embodiments.

As above, the first inorganic material is optionally (and typically) asemiconductor, as is the second inorganic material. The materials can beany of those noted for the preceding embodiments. As with the precedingembodiments, the first population of nanostructures can includenanocrystals that comprise a core of the first inorganic material and ashell of the second inorganic material, e.g., any of those noted above.The nanocrystals are optionally fused, partially fused, and/or sintered,as in the preceding embodiments, providing for partial and/or directelectrical contact between nanostructure cores or shells as noted above.

In one such embodiment, the first population of nanostructures comprisesnanocrystals comprising the first inorganic material, and thephotoactive layer further comprises a second population of nanocrystalscomprising nanocrystals which comprise the second inorganic material,e.g., wherein adjacent nanocrystals are in at least partial directelectrical contact with each other.

The nanocrystals of the first population and the nanocrystals of thesecond population are optionally intermixed in the photoactive layer.Alternately, the photoactive layer comprises at least a first sublayerand a second sublayer, wherein the first sublayer comprises the firstpopulation of nanocrystals and the second sublayer comprises the secondpopulation of nanocrystals.

As in the embodiments above, the photoactive layer optionally comprisesat least two active sublayers, each of which optionally includes aplurality of nanocrystals of at least one nanocrystal type. As noted inthe examples above, and as applicable to these embodiments as well, atleast one of the at least two sublayers optionally comprises an n-typesublayer and/or a p-type sublayer, e.g., in any of the arrangementsnoted above (e.g., blended or separate p and/or n nanostructures, inblended or separate layers, optionally separated by one or morejunctions).

For this set of embodiments, the photoactive layer can simply consist ofnanocrystals, e.g., fused, partially fused or sintered nanocrystals.However, a polymer or matrix can also be included in the photoactivelayer. For example, the photoactive layer can include a conductive ornon-conductive polymer. The layer can include such a polymer matrix inwhich nanocrystals are disposed, e.g., in which nanocrystals of thefirst and/or second populations are electrically coupled to the polymerto provide for conductance of holes or electrons (e.g., to theelectrodes of the device). However, such a polymer or other matrixcomponent is not required, and the photoactive layer is optionallysubstantially free of any conductive and/or non-conductive polymer.

While the orientation of the first set of embodiments is not required inthis second set of embodiments, the noted orientation is, optionally, afeature of the second set of embodiments as well. For example, thenanostructures of the first population can each have at least oneelongated section oriented predominantly normal to at least the firstplane. Similarly, the second population of nanostructures can displaythe same (or a different) orientation.

In addition to photovoltaic devices, the invention further providescompositions useful for constructing photovoltaic and other devices.These compositions can include or be comprised within photovoltaic orother devices (e.g., LEDs, dual crystal devices, etc). For example, acomposition comprising a first population of nanostructures and a secondpopulation of nanostructures is provided. The first population comprisesnanostructures comprising a first material, and the second populationcomprises nanostructures comprising a second material different from thefirst material.

As in the preceding embodiments, the nanostructures optionally includenanocrystals and/or nanowires. As in the preceding devices, thenanostructures can also comprise single-crystal nanostructures,double-crystal nanostructures, polycrystalline nanostructures, oramorphous nanostructures. Typically, the first material of thenanostructures is a first inorganic material and the second material isa second inorganic material. Optionally, the first material comprises afirst semiconductor and the second material comprises a secondsemiconductor. For example, the first material optionally comprises ann-type semiconductor and the second material optionally comprises ap-type semiconductor. As in the photovoltaic devices noted above, thefirst and second materials can exhibit a type II band offset energyprofile. However, in certain non-photovoltaic applications (e.g., LEDs),the first and second materials exhibit other energy profiles, e.g., atype I band offset energy profile. As in the photovoltaic devices notedabove, adjacent nanostructures are optionally in at least partial (ordirect) electrical contact with each other.

Optionally, the nanostructures of the first population and thenanostructures of the second population are in separate layers, or areintermixed. The populations can be located in one or more discrete ormixed regions, zones, layers, or the like. The composition can be formedinto a film.

The composition optionally includes at least a first sublayer and asecond sublayer. In such embodiments, the first sublayer optionallyincludes the first population of nanostructures and the second sublayeroptionally includes the second population of nanostructures. In severalapplications, e.g., photovoltaic applications, the film or othercomposition is disposed between two electrode layers (or betweenadditional electrode layers). As with the photovoltaic applicationsnoted above, the nanostructures of the first and/or second populationsare optionally fused, partially fused, and/or sintered (or not fused,partially fused or sintered).

The composition optionally includes a conductive polymer. For example,the composition optionally includes a polymer matrix in whichnanostructures are disposed. Optionally, any feature of thenanostructures (surface, core, shell, etc.) are electrically coupled tothe polymer. The polymer matrix can alternately or additionally benon-conductively coupled to the nanostructures. Alternately, thecomposition can be substantially free of conductive and/ornon-conductive polymer.

In addition to the devices and compositions noted above, related methodsand systems are also provided. In a first class of methods, methods ofproducing a photovoltaic device are provided. In the methods, a firstplanar substrate having a first conductive layer disposed thereon isprovided. The first substrate is coated with a photoactive matrix thatexhibits a type II band offset energy profile. The photoactive matrixincludes at least a first population of elongated semiconductornanostructures, comprising longitudinal axes, to provide a photoactivelayer. The semiconductor nanostructures are oriented such that theirlongitudinal axes are predominantly oriented normal to the first planarsubstrate. A second conductive layer is laminated onto the photoactivelayer.

Essentially any of the features of the devices and compositions notedabove can be included in these methods. For example, the nanostructuresoptionally include a nanocrystal, a nanowire, a single-crystalnanostructure, a double-crystal nanostructure, a polycrystallinenanostructure, and/or an amorphous nanostructure. The nanostructures canbe, e.g., any of those noted above, as can any other components of thematrix.

The method optionally includes providing a blocking layer on the firstsubstrate prior to coating the first substrate with a photoactivematrix. Similarly, the method can include providing a blocking layer onthe photoactive layer prior to laminating the second conductive layeronto the photoactive layer. One or more sealing layers can optionally beprovided over opposing surfaces of the photovoltaic device in additionto first substrate and second conductive layer, whereby the one or moresealing layers hermetically seal the photovoltaic device.

A second class of methods of producing a photovoltaic device is alsoprovided. In the methods, a first planar substrate having a firstconductive layer disposed thereon is provided. The first substrate iscoated with a composition that comprises a population of nanostructuresto provide a photoactive layer. The nanostructures comprise a core of afirst material and a shell of a second material different from the firstmaterial. The nanostructures are fused, partially fused, and/orsintered, and a second conductive layer is laminated onto thephotoactive layer.

As in the preceding class of methods, essentially any of the features ofthe devices and compositions noted above can be included in thesemethods. For example, the nanostructures optionally include ananocrystal, a nanowire, a single-crystal nanostructure, adouble-crystal nanostructure, a polycrystalline nanostructure, and/or anamorphous nanostructure. The nanostructures can be, e.g., any of thosenoted above, as can any other components of the matrix. Typically, thefirst material is a first inorganic material and the second material isa second inorganic material. For example, the first material optionallyincludes a first semiconductor and the second material optionallyincludes a second semiconductor.

As in the preceding class of methods, the methods optionally includeproviding a blocking layer on the first substrate prior to coating thefirst substrate with the composition. Similarly, the methods optionallyinclude providing a blocking layer on the photoactive layer prior tolaminating the second conductive layer onto the photoactive layer. Oneor more sealing layers can be provided over opposing surfaces of thephotovoltaic device, whereby the one or more sealing layers hermeticallyseal the photovoltaic device.

In a third class of method embodiments, methods of producing a layereddevice comprising a first population of nanostructures and a secondpopulation of nanostructures are provided. The first population includesnanostructures comprising a first material, and the second populationcomprises nanostructures comprising a second material different from thefirst material. The methods include providing a first substrate, andcoating the first substrate with a composition comprising the firstpopulation of nanostructures to provide a first layer.

As in the preceding classes of methods, essentially any of the featuresof the devices and compositions noted above can be included in thesemethods. For example, the nanostructures optionally include ananocrystal, a nanowire, a single-crystal nanostructure, adouble-crystal nanostructure, a polycrystalline nanostructure, and/or anamorphous nanostructure. The nanostructures can be, e.g., any of thosenoted above, as can any other components of the matrix. Typically, thefirst material is a first inorganic material and the second material isa second inorganic material. For example, the first material optionallyincludes a first semiconductor and the second material optionallyincludes a second semiconductor.

Coating the first substrate with a composition comprising the firstpopulation of nanostructures optionally includes coating the firstsubstrate with a composition comprising a mixture of the first andsecond populations of nanostructures. This provides a first layer inwhich the nanostructures of the first and second populations areintermixed.

The method optionally further includes coating the first substrate witha composition comprising the second population of nanostructures, toprovide a second layer. For example, the second population ofnanostructures can be disposed on the first substrate.

The first substrate can be planar or non-planar, e.g., any of thegeometries noted for the devices above. The method can be used toproduce a photovoltaic device, a luminescent device, or the like. Afirst conductive layer can be disposed on the first planar substrate anda second conductive layer can optionally be layered onto the first (orsecond) layer.

In the methods, a blocking layer is optionally provided on the firstsubstrate prior to coating the first substrate with the compositioncomprising the first population of nanostructures. Similarly, a blockinglayer can be provided on the first layer prior to laminating the secondconductive layer onto the first (or second) layer. As in the methodsabove, one or more sealing layers can be provided over opposing surfacesof the device, whereby the one or more sealing layers optionallyhermetically seal the device.

Systems for fabricating a photovoltaic device are also provided. Forexample, such a system can include a source of a first substrate layer,having a first conductive surface. The system can also include aconveyor system for conveying the first substrate layer. A source of aphotoactive matrix can be fluidly coupled to a layer deposition system,the layer deposition system being at least partially disposed over thesubstrate conveyor system, to provide a layer of photoactive matrix onthe first substrate layer. A source of a second conductive material iscoupled to the layer deposition system positioned over the substrateconveyor system for depositing a layer of the second conductive surfacematerial onto a layer of photoactive matrix deposited on the firstsubstrate layer.

In one example embodiment, the source of first substrate materialincludes a rolled sheet of the first substrate material. The source offirst substrate material optionally includes a source of firstconductive material and a deposition system for depositing the firstconductive material onto the first substrate material to provide thefirst conductive surface.

The layer deposition system can include, e.g., a doctor-blade, a screenprinting system, an ink-jet printing system, a dip coating system, asheer coating system, a tape casting system, a film casting system, or acombination thereof.

In a related class of embodiments, a system for fabricating a layereddevice is also provided. The system includes a first population ofnanostructures and a second population of nanostructures. The firstpopulation comprises nanostructures comprising a first material, and thesecond population comprises nanostructures comprising a second materialdifferent from the first material. The system also includes a source ofa first substrate layer; a conveyor system for conveying the firstsubstrate layer; and, a source of a composition comprising the first andsecond populations of nanostructures, fluidly coupled to a layerdeposition system, the layer deposition system being at least partiallydisposed over the substrate conveyor system, to provide a layer in whichthe nanostructures of the first and second populations are intermixed onthe first substrate layer.

As in the devices, systems and methods above, the nanostructures cancomprise nanocrystals or any of the other nanostructures noted herein orotherwise applicable. For example, the nanostructures can includenanocrystals and/or nanowires. The nanostructures can include, e.g., asingle-crystal nanostructure, a double-crystal nanostructure, apolycrystalline nanostructure, and/or an amorphous nanostructure.Similarly, the first and second material can be any of those notedabove, e.g., the first material can be a first inorganic material andthe second material can be a second inorganic material. Optionally, thefirst substrate layer has a first conductive surface.

The system optionally includes a source of a second conductive materialcoupled to the layer deposition system, e.g., positioned over thesubstrate conveyor system for depositing a layer of the secondconductive material onto the layer of intermixed nanostructuresdeposited on the first substrate layer.

As in the systems above, the source of first substrate material canoptionally include a rolled sheet of first substrate material.Similarly, the source of first substrate material can further include asource of first conductive material and a deposition system fordepositing the first conductive material onto the first substratematerial to provide a first conductive surface. The layer depositionsystem can be any of those noted above.

In an additional related class of system embodiments, systems forfabricating layered devices comprising a first population ofnanostructures and a second population of nanostructures, which firstpopulation comprises nanostructures comprising a first material, andwhich second population comprises nanostructures comprising a secondmaterial different from the first material, are provided. The systemincludes a source of a first substrate layer; a conveyor system forconveying the first substrate layer; a source of a first compositioncomprising the first population of nanostructures fluidly coupled to alayer deposition system, the layer deposition system being at leastpartially disposed over the substrate conveyor system, to provide afirst layer; and, a source of a second composition comprising the secondpopulation of nanostructures fluidly coupled to the layer depositionsystem, the layer deposition system being at least partially disposedover the substrate conveyor system, to provide a second layer.

The system features noted for the preceding embodiments with respect tonanostructure types, first and/or second material types, conductivesurfaces on the substrate layer(s), sources of first or secondconductive materials, deposition systems for depositing the materials,use of rolled sheets, layering systems, etc. are applicable here aswell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a cross sectional view of asimplified photovoltaic device according to the present invention.

FIG. 2 schematically illustrates an energy diagram of elements of ananocomposite photovoltaic device according to the present invention.

FIG. 3A-3D schematically illustrates a comparison of differentnanocomposite compositions and configurations in an active layer, andelectron conduction therethrough.

FIG. 4A-4B schematically illustrates a cross sectional view ofpositioned nanocrystals in an active layer.

FIG. 5 is a plot of absorption spectra of CdSe nanorods of varyingdiameters.

FIG. 6 schematically illustrates a sealed photovoltaic device.

FIG. 7 schematically illustrates an alternate configuration of aphotovoltaic device that includes a meshed, perforated, or partiallytransparent or translucent electrode on its upper surface.

FIGS. 8A-8C schematically illustrate several alternate configurations offlexible or conformable photovoltaic devices according to the presentinvention.

FIG. 9 schematically illustrates a system and process for highthroughput fabrication of photovoltaic devices according to theinvention.

FIG. 10 is a schematic illustration of a cross sectional view of asimplified photovoltaic device according to the present invention.

FIG. 11 is a schematic illustration of a cross sectional view of asimplified photovoltaic device according to the present invention.

FIG. 12 is a schematic illustration of a cross sectional view of asimplified photovoltaic device according to the present invention.

DEFINITIONS

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which the invention pertains. The following definitionssupplement those in the art and are directed to the current applicationand are not to be imputed to any related or unrelated case, e.g., to anycommonly owned patent or application. Although any methods and materialssimilar or equivalent to those described herein can be used in thepractice for testing of the present invention, the preferred materialsand methods are described herein. Accordingly, the terminology usedherein is for the purpose of describing particular embodiments only, andis not intended to be limiting.

As used in this specification and the appended claims, the singularforms “a,” “an” and “the” include plural referents unless the contextclearly dictates otherwise. Thus, for example, reference to “ananostructure” includes a plurality of nanostructures, and the like.

An “aspect ratio” is the length of a first axis of a nanostructuredivided by the average of the lengths of the second and third axes ofthe nanostructure, where the second and third axes are the two axeswhose lengths are most nearly equal each other. For example, the aspectratio for a perfect rod would be the length of its long axis divided bythe diameter of a cross-section perpendicular to (normal to) the longaxis.

The terms “crystalline” or “substantially crystalline”, when used withrespect to nanostructures, refer to the fact that the nanostructurestypically exhibit long-range ordering across one or more dimensions ofthe structure. It will be understood by one of skill in the art that theterm “long range ordering” will depend on the absolute size of thespecific nanostructures, as ordering for a single crystal cannot extendbeyond the boundaries of the crystal. In this case, “long-rangeordering” will mean substantial order across at least the majority ofthe dimension of the nanostructure. In some instances, a nanostructurecan bear an oxide or other coating, or can be comprised of a core and atleast one shell. In such instances it will be appreciated that theoxide, shell(s), or other coating need not exhibit such ordering (e.g.it can be amorphous, polycrystalline, or otherwise). In such instances,the phrase “crystalline,” “substantially crystalline,” “substantiallymonocrystalline,” or “monocrystalline” refers to the central core of thenanostructure (excluding the coating layers or shells). The terms“crystalline” or “substantially crystalline” as used herein are intendedto also encompass structures comprising various defects, stackingfaults, atomic substitutions, and the like, as long as the structureexhibits substantial long range ordering (e.g., order over at leastabout 80% of the length of at least one axis of the nanostructure or itscore). In addition, it will be appreciated that the interface between acore and the outside of a nanostructure or between a core and anadjacent shell or between a shell and a second adjacent shell maycontain non-crystalline regions and may even be amorphous. This does notprevent the nanostructure from being crystalline or substantiallycrystalline as defined herein.

The term “monocrystalline” when used with respect to a nanostructureindicates that the nanostructure is substantially crystalline andcomprises substantially a single crystal. When used with respect to ananostructure heterostructure comprising a core and one or more shells,“monocrystalline” indicates that the core is substantially crystallineand comprises substantially a single crystal.

The term “heterostructure” when used with reference to nanostructuresrefers to nanostructures characterized by at least two different and/ordistinguishable material types. Typically, one region of thenanostructure comprises a first material type, while a second region ofthe nanostructure comprises a second material type. In certainembodiments, the nanostructure comprises a core of a first material andat least one shell of a second (or third etc.) material, where thedifferent material types are distributed radially about the long axis ofa nanowire, a long axis of an arm of a branched nanocrystal, or thecenter of a nanocrystal, for example. A shell need not completely coverthe adjacent materials to be considered a shell or for the nanostructureto be considered a heterostructure; for example, a nanocrystalcharacterized by a core of one material covered with small islands of asecond material is a heterostructure. In other embodiments, thedifferent material types are distributed at different locations withinthe nanostructure; e.g., along the major (long) axis of a nanowire oralong a long axis of arm of a branched nanocrystal. Different regionswithin a heterostructure can comprise entirely different materials, orthe different regions can comprise a base material (e.g., silicon)having different dopants or different concentrations of the same dopant.

A “nanocrystal” is a nanostructure that is substantiallymonocrystalline. A nanocrystal thus has at least one region orcharacteristic dimension with a dimension of less than about 500 nm,e.g., less than about 200 nm, less than about 100 nm, less than about 50nm, or even less than about 20 nm. Nanocrystals can be substantiallyhomogeneous in material properties, or in certain embodiments can beheterogeneous (e.g. heterostructures). The term “nanocrystal” isintended to encompass substantially monocrystalline nanostructurescomprising various defects, stacking faults, atomic substitutions, andthe like, as well as substantially monocrystalline nanostructureswithout such defects, faults, or substitutions. In the case ofnanocrystal heterostructures comprising a core and one or more shells,the core of the nanocrystal is typically substantially monocrystalline,but the shell(s) need not be. The nanocrystals can be fabricated fromessentially any convenient material or materials. In one aspect, each ofthe three dimensions of the nanocrystal has a dimension of less thanabout 500 nm, e.g., less than about 200 nm, less than about 100 nm, lessthan about 50 nm, or even less than about 20 nm. Examples ofnanocrystals include, but are not limited to, substantially sphericalnanocrystals, branched nanocrystals, and substantially monocrystallinenanowires, nanorods, nanodots, quantum dots, nanotetrapods, tripods,bipods, and branched tetrapods (e.g., inorganic dendrimers).

A “nanostructure” is a structure having at least one region orcharacteristic dimension with a dimension of less than about 500 nm,e.g., less than about 200 nm, less than about 100 nm, less than about 50nm, or even less than about 20 nm. Typically, the region orcharacteristic dimension will be along the smallest axis of thestructure. Examples of such structures include nanowires, nanorods,nanotubes, branched nanocrystals, nanotetrapods, tripods, bipods,nanocrystals, nanodots, quantum dots, nanoparticles, branched tetrapods(e.g., inorganic dendrimers), and the like. Nanostructures can besubstantially homogeneous in material properties, or in certainembodiments can be heterogeneous (e.g. heterostructures). Nanostructurescan be, e.g., substantially crystalline, substantially monocrystalline,polycrystalline, amorphous, or a combination thereof. In one aspect,each of the three dimensions of the nanostructure has a dimension ofless than about 500 nm, e.g., less than about 200 nm, less than about100 nm, less than about 50 nm, or even less than about 20 nm.

A “nanowire” is a structure (typically, a nanostructure) that has oneprinciple axis that is longer than the other two principle axes and thathas an aspect ratio greater than about 10 (e.g., greater than about 20,greater than about 50, or greater than about 100, or even greater thanabout 10,000). The diameter of a nanowire is typically less than about500 nm, preferably less than about 200 nm, more preferably less thanabout 150 nm, and most preferably less than about 100 nm, about 50 nm,or about 25 nm, or even less than about 10 nm or about 5 nm. The lengthof a nanowire is typically greater than about 100 nm, e.g., greater than200 nm, greater than 500 nm, or even greater than 1000 nm. The nanowiresof this invention can be substantially homogeneous in materialproperties, or in certain embodiments can be heterogeneous (e.g.nanowire heterostructures). The nanowires can be fabricated fromessentially any convenient material or materials, and can be, e.g.,substantially crystalline, substantially monocrystalline,polycrystalline, or amorphous. Nanowires can have a variable diameter orcan have a substantially uniform diameter, that is, a diameter thatshows a variance less than about 20% (e.g., less than about 10%, lessthan about 5%, or less than about 1%) over the region of greatestvariability and over a linear dimension of at least 5 nm (e.g., at least10 nm, at least 20 nm, or at least 50 nm). Typically the diameter isevaluated away from the ends of the nanowire (e.g. over the central 20%,40%, 50%, or 80% of the nanowire). A nanowire can be straight or can bee.g. curved or bent, over the entire length of its long axis or aportion thereof. In certain embodiments, a nanowire or a portion thereofcan exhibit two- or three-dimensional quantum confinement. Nanowiresaccording to this invention can expressly exclude carbon nanotubes, and,in certain embodiments, exclude “whiskers” or “nanowhiskers”,particularly whiskers having a diameter greater than 100 nm, or greaterthan about 200 nm.

DETAILED DESCRIPTION

Semiconductor nanocrystals, generally referred to as nanodots, nanorods,nanowires, etc., with their photoluminescent and electroluminescentproperties have been the subject of a great deal of research into newlabeling and luminescent display technologies.

In operation, when a semiconductor nanocrystal is exposed to light, theimpact of photons of a given wavelength produces an excited state thatis characterized by the displacement of an electron from its orbital.The resulting separated charges, an electron and an electron acceptor orhole, also together termed an “exciton,” would then typically recombine.When the electron and hole recombine, they emit light at a wavelengthcharacteristic of the energy released. By varying the characteristics ofthe material, one could adjust the wavelength of emitted light.

Thus, to date, nanorod and nanodot research and development hasprimarily exploited this charge recombination within these nanomaterialsto take advantage of the luminescent results. However, it has been shownthat by separating the charges in an exciton, e.g., an electron from ahole, one can exploit the resulting electrical potential, thus derivingelectric current from optical energy. See, e.g., Huynh, et al., Science295(5564):2426-2427 (2002), Huynh, et al., Adv. Materials 11(11):923(1999), Greenham et al., Phys. Rev. B 54(24):17628-17637 (1996), andU.S. Pat. No. 6,239,355, each of which is hereby incorporated herein byreference in its entirety for all purposes. In particular, photovoltaicdevices have been reported that exploit a nanocrystal composite activelayer in the conversion of light energy to electrical energy.

The present invention provides additional photovoltaic devicescomprising a nanocomposite active layer, as well as photovoltaic andother devices in which the active layer comprises nanocrystals that arenot necessarily part of a composite. Related compositions, methods, andsystems are also provided.

I. Active Layers

A. Nanocomposite Active Layers

One general class of embodiments provides photovoltaic devices in whichthe active layer comprises nanostructures (e.g., nanocrystals,nanowires, single-crystal nanostructures, double-crystal nanostructures,polycrystalline nanostructures, or amorphous nanostructures). Briefly,when light impinges upon the nanocrystal component of the active layer,it is absorbed by the nanocrystal creating an exciton within thenanocrystal. By conducting the electron away from the hole, one createsan electric potential that can be exploited. In the case ofnanocomposite photoactive layers, this is accomplished by disposing thenanocrystal component in a conductive polymer matrix that is able todonate an electron to the nanocrystal (or conduct the hole away from thenanocrystal). Because the nanocrystals comprise semiconductive material,the charge mobility (movement of the electrons) in the nanocrystalcomponent is sufficiently fast at conducting the electron away from thehole in the polymer matrix, so as to avoid recombination.

Once charges are separated, the electrons are selectively conductedthrough the active layer unidirectionally, e.g., out of the active layerthrough one electrode and back into the active layer through anotherelectrode, providing useful current flow in the intervening circuit.Unidirectional conduction is also generally discussed in terms ofmovement of electrons in one direction and movement of holes in theother direction. The operation of one of these devices is schematicallyillustrated in FIG. 1. In particular, the general device structure 100comprises an active layer 102 that is comprised of a material thatdisplays or otherwise exhibits charge carrying properties having a typeII band offset. As shown, the material meeting this criterion includes ananocrystal component 104 and a polymer component 106. The active layer102 is sandwiched between first and second electrodes 108 and 110,respectively. As shown, electrode 110 is disposed on a separatesubstrate 116, although the electrode(s) and substrate may be oneintegral unit. At least one of the electrodes, e.g., electrode 110, isprovided as a transparent electrode or electrode layer. When light (asindicated by arrow 112) impinges upon the nanocrystal component 104, itcreates an exciton which passes a hole (θ) into the polymer matrix 108,and conducts the electron (e−) along the nanocrystal 104 (as indicatedby the dashed line). The electron is conducted to electrode 108 whilethe hole is carried to electrode 106. The resulting current flow, e.g.,in the direction of arrows 115 is then exploited, e.g., in load/device114.

FIG. 2 schematically illustrates an energy diagram of the functioning ofa nanocomposite photovoltaic device, as described herein, which dictatesphotoactivated charge separation, e.g., flow of holes toward oneelectrode and electrons toward the other electrode. By way of example,in certain cases, these energy plots show how electrons are retained inthe nanocrystal and are conducted toward one electrode, while holes aretransferred out the nanocrystal, e.g., into the polymer matrix, andconducted toward the other electrode. Shown is a plot of the workfunctions of the various components of the device, and the transfer ofcharges, e.g., electrons or holes among those components. The activelayer is indicated in zones II and III, while the electrodes are shownin zones I and IV. As can be seen, the work functions of the variouscomponents are selected to obtain substantially unidirectional flow ofelectrons and holes, resulting in charge separation to the opposingelectrodes. As can be seen, the active layer provides components thathave a type II band offset, e.g., electrons and holes are flowing inopposite directions between zones II and III, achieving initial chargeseparation. The work functions for electron conduction and holeconduction for one component of the matrix, e.g., the nanocrystalsand/or the core, are offset in the same direction, from those of theother component, e.g., the conducting polymer and/or the shell. Onceseparated, the work functions of the opposing electrodes further providefor charge separation, e.g., electrons flow preferentially to oneelectrode while holes flow to the other.

While prior researchers have demonstrated the basic functioning of ananocomposite photovoltaic cell, the previously reported work shows thatthere is ample room for improvement of these devices, includingincreasing efficiency, e.g., from approximately 7%, increasingdurability and manufacturability. In accordance with the presentinvention, a number of improvements are provided to the basicnanocomposite photovoltaic device that produces substantial improvementsin efficiency across the range of light exposures, improvements inproduct robustness, as well as a number of other commercially valuableimprovements.

The basic architecture of the photovoltaic devices of the invention issubstantially as described with reference to FIG. 1. In particular,these devices typically include an active or photoactive layer 102 thatis sandwiched between two electrically conductive or electrode layers108 and 110. For ease of discussion, the electrodes are referred to asbeing planar or having a planar orientation. However as will beappreciated, electrodes and entire devices may take on a variety ofshapes, including curved, corrugated, coiled or other decidedlynon-planar architectures. As such, reference to the plane of theelectrodes refers to the plane of the immediate region being referencedand may include the plane that is tangential to the actual electrodesurface in this region. In one general class of embodiments, thephotoactive layer is comprised of a nanocomposite material that providesfor unidirectional electron movement, or current flow, upon exposure ofthe active layer to light. As used in the present invention, thenanocomposite typically includes a semiconductor nanostructure (e.g.,nanocrystal) component that is disposed in a conductive matrix, andpreferably a conductive polymer matrix, where the polymer matrixfunctions as a hole conductor while the nanostructures (e.g.,nanocrystals) function as the electron conductor. With reference to FIG.2, the nanocrystals would function as the zone II component, while theconducting polymer would be the zone III component.

1. Semiconductor Nanocrystals

Nanostructures, especially semiconductor nanocrystals, as noted above,have been described in great detail, previously. As used herein,semiconductor nanocrystals include a wide range of different materialsthat exist as nanoparticles, e.g., having at least one cross sectionaldimension of less than about 500 nm, and preferably, less than 100 nm.These nanocrystals may be comprised of a wide range of semiconductivematerials, including for example, group III-V, group II-VI and group IVsemiconductors or alloys of these materials. In particularly preferredaspects, CdSe, CdTe, InP, InAs, CdS, ZnS, ZnO, ZnSe, PbSe, PbS, ZnTe,HgTe, GaN, GaP, GaAs, GaSb, InSb, Si, Ge, AlAs, AlSb or PbTesemiconductors or their alloys are used as at least a portion of thenanocrystal component.

Although generally described in terms of nanocrystals, in accordancewith most preferred aspects of the invention, the nanocrystal componentof the active layer will be comprised, at least in part, and preferablyin substantial part, of elongated nanocrystals or nanorods, that includeaspect ratios (length:diameter) of 5, 10 or greater, or threedimensional nanostructures that include nanorod-like components, e.g.,four branched or tetrahedral structures also termed “nanotetrapods.” Theuse of rod-like structures in place of substantially sphericalstructures, e.g., quantum dots, provides substantial advantages in termsof charge separation. In particular, and without being bound to aparticular theory of operation, because of their aspect ratio, nanorodsare able to separate an electron from a hole, e.g., the electron isconducted along the length of the rod, while the hole is transferred theshort distance of the radius of the rod to the surrounding matrix.Particularly preferred nanorods and nanotetrapods and their methods ofproduction are described in detail in, e.g., Peng et al, Nature404(6773):59-61 (2000), Manna et al., J. Am. Chem. Soc.122(51):12700-12706 (2000), and Manna et al., J. Am. Chem. Soc.124(24):7136-7145 (2002), each of which is incorporated herein byreference in its entirety for all purposes.

The semiconductor nanocrystals may optionally comprise additionalelements to enhance their function within the active layer. For example,nanocrystals may be dye sensitized to increase light absorption and/orcharge injection into the nanocrystal component. Examples of such dyesinclude those described in U.S. Pat. No. 6,245,988, published PCTApplication Nos. WO 94/04497 and 95/29924, each of which is incorporatedherein by reference in its entirety for all purposes, where rutheniumbased dyes are provided in conjunction with crystalline components toenhance light absorption and charge injection.

In a number of preferred aspects, the nanocrystal component willcomprise a nanoheterostructure. In particular, in certain preferredaspects, the nanocrystal component will comprise a core-shell structurewhere the core portion of the crystal comprises a first material, and isoverlaid with a coating or shell of another material. Core-shellnanostructures have been described previously (see, U.S. Pat. Nos.6,207,229 and 6,322,901, incorporated herein by reference in theirentirety for all purposes). Particularly preferred are core-shellnanocrystals, e.g., nanorods, having a type-II band offset between thecore and the shell, where the core transports one portion of theexciton, e.g., the electrons, while the shell transports the other,e.g., holes. As a result, the core-shell nanorod efficiently maintainscharge separation and carries out charge conduction, both as toelectrons and holes. With reference to FIG. 2, the core-shellnanocrystals described herein would function as both the zone II andzone III component of the active layer. By utilizing these materials,one can substantially, if not entirely, eliminate the need for theconducting polymer component of the active layer of any device. Examplesof core-shell materials that possess such type 2 offsets include, e.g.,CdSe—CdTe nanorods and InP—GaAs nanorods. Alternatively, longitudinalheterostructures may also be included as the nanocrystal component ofthe active layer, e.g., those with one or more heterojunctions along thelongitudinal axis of the crystal, e.g., as described in Published PCTApplication Nos. WO 02/080280 and WO 02/17362, the full disclosures ofwhich are hereby incorporated herein by reference in their entirety forall purposes.

Use of core-shell nanocrystals provides additional advantages of beingable to eliminate conductive polymers from the active layer. Inparticular, the inclusion of conductive polymers in carrying out thecharge separation operation is believed to result in some lostefficiency associated with transfer of charge from the crystal to thepolymer. This may be addressed as described elsewhere herein, e.g.,through the inclusion of linking chemistries between crystal andpolymer. More concerning in the use of conductive polymers is theirsusceptibility to oxidation, photooxidation, or other environmentalconditions, which can severely limit their durability in many commercialapplications if not hermetically sealed. Because inorganic nanocrystalsdo not have these sensitivities, their use provides for drasticsimplification of packaging and manufacturing methods.

2. Polymer Matrix

A variety of different conductive matrices are useful as the supportingand conductive matrix of the active layer. Conductive polymers aregenerally described in T. A. Skatherin (ed.), Handbook of ConductingPolymers I, which is incorporated herein by reference in its entiretyfor all purposes. Examples of preferred conductive polymer matricesinclude, e.g., poly(3-hexylthiophene)(P3HT), poly[2-methoxy,5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene](MEH-PPV), poly(phenylenevinylene)(PPV), and polyaniline. Because of the application of thedevices described herein, it will be appreciated that preferred polymermatrices will be light stable, and depending upon the packaging of thedevices, oxygen stable as well.

One aspect of active layers where it is believed that efficiencies canbe increased is in the charge separation between a semiconductornanocrystal and a conductive polymer matrix in an active layer. Inparticular, it is believed that at least some of the inefficiencies ofnanocomposite photovoltaic active layers derive from recombination ofexcitons within the nanocrystal. Accordingly, without being bound to aparticular theory of operation, it is expected that by providing a moreefficient conduction of holes from the nanocrystal to the polymermatrix, or among the layers of a core-shell nanocrystal, one couldobtain a consequent improvement in efficiency of the overall device.

In accordance with certain aspects of the present invention, chargeseparation efficiency is at least partially addressed by coupling theouter surface or shell of the nanocrystal to the polymer matrix. Thecoupling comprises an electrically conductive coupling that provides amore direct route of conduction from the nanocrystal to the polymermatrix. Typically, such couplings may comprise any of a variety ofcovalent chemical, ionic, hydrophobic/hydrophilic interactions betweenthe polymer and the nanocrystal, either directly or through one or morelinking molecules. Examples of useful polymer/nanocrystal linkagesinclude, e.g., modifying the polymer side chains, e.g., polymers likeP3HT, PPV or their derivatives, to directly bind nanocrystal sidechains, i.e., adding phosphonic acid groups, phosphine oxides,phosphine, amine, thiol or other chemistries that will couple to thepassivated, partially passivated, and/or unpassivated atoms (e.g., tothe cation or anion groups) present on a nanocrystal surface. As notedabove, and as should be readily apparent, where the electron carrier andhole carrier are associated as the same structure, e.g., as a core-shellnanocrystal, charge separation efficiencies would be further expected toincrease, which may be used in conjunction with or in certain preferredaspects, in lieu of a chemical linkage to a polymer matrix. Examples ofuseful approaches to surface chemistries for linkage to polymers orotherwise to enhance charge injection and/or extraction fromnanocrystals are described in substantial detail in Provisional U.S.Patent Application No. 60/452,232 (Attorney Docket No. 40-002700US),filed Mar. 4, 2003, and U.S. patent application Ser. No. 10/656,910(Attorney Docket No. 40-002710US), filed on Sep. 4, 2003, which areincorporated herein by reference in their entirety for all purposes.

Alternatively, ligands may be coupled to the outer surface of thenanocrystal via conventional chemistries, e.g., as described elsewhereherein and in, e.g., Provisional U.S. Patent Application No. 60/392,205,filed Jun. 27, 2002, which is incorporated herein by reference in itsentirety for all purposes. The ligand is then coupled at a secondposition to the polymer matrix, e.g., via conventional linkingchemistries.

Alternative or additional polymer matrix modifications may also beprovided in addition to or instead of those described above, includingdoping of conducting polymers with charged groups and/or controlledoxidation of the conducting polymer to permit better charge conductionthrough the polymer.

3. Active Layer Configuration

As noted above, conduction of electrons is much more rapid through thesemiconductor nanocrystals (or other semiconductor nanostructures) thanthrough the surrounding polymer matrix. In cases where an electron isconducted or hops from one crystal to another, there is a consequentloss in energy associated with that hopping. By minimizing lossesassociated with electron hopping, one can improve the efficiency of theoverall device. In order to minimize hopping, therefore, it is desirableto provide as direct a conductive path through the active layer, aspossible, e.g., a path with fewer required crystal to crystal hops. Inthe case of spherical or near spherical nanocrystals, this is bestaccomplished by providing the nanocrystal component as substantially amonolayer disposed on one electrode. While this provides an advantage incharge conduction, it carries the disadvantage of substantially reducedlight absorption from a single thin layer of nanocrystals.

The use of nanorods or nanotetrapods, however, provides significantadvantages in this respect. In particular, because these nanorods andnanotetrapods have elongated sections and maintain quantum confinementin one dimension, they provide both increased light absorption, e.g.,they absorb well along the entire length of the rod) and a longerconductive path, and thus have the potential for more efficient chargeconduction. By orienting this longer conductive path to bridge the spacebetween the electrodes, e.g., normal to the plane of the electrodes, onecan further provide a more direct and longer conductive path forelectrons.

Accordingly, in preferred aspects, the active layers described hereinemploy nanostructures that provide an elongated conductive path, whichpath is oriented predominantly normal to the plane of the two opposingelectrodes of the device. As noted above, this can be accomplished byproviding elongated nanorods that are oriented such that theirlongitudinal axes are predominantly normal to the plane of theelectrodes. By “oriented predominantly normal to the plane” is generallymeant that the average longitudinal dimension of the collection ofnanorods within the active layer is oriented more normal to the plane ofthe two electrodes than would be the case of a completely randomlyoriented collection of nanorods.

In preferred aspects, the nanorod component of the active layer is evenmore stringently oriented, such that at least 50% of the nanorods in acollection of nanorods are oriented such that they include elongatedsections that have their longitudinal axes oriented within 45° of normalto the plane of the electrodes. In more preferred aspects, at least 80%of the nanorods in the active layer are oriented such that they haveelongated sections that have their longitudinal axes are within 45° ofnormal to the plane of the electrodes, and in still further aspects, atleast 90% or even 95% of the nanorods are so oriented. In someembodiments, the orientation may be more toward normal, with the greaterpercentage of rods, e.g., the various percentages set forth above, beingoriented such that their longitudinal axes are within at least 30° ofnormal to the plane of the electrodes, and in some cases oriented within20° or even 15° or less to the plane of the electrodes.

In one aspect, the present invention provides compositions such asphotoactive layers having aligned and/or arranged nanostructureelements, as well as methods for preparing aligned and/or arrangednanostructures. Any of a number of nanostructures (or combination ofnanostructures) can be used in the compositions and methods of thepresent invention, including, but not limited to, nanodots, nanospheres,nanorods, nanowires, nanoribbons, nanotetrapods, various branchedstructures (e.g., dendrimer branching structures), quantum dots,nanodots, and the like.

In one aspect, the present invention provides compositions having aplurality of non-randomly oriented nanostructures in a matrix, e.g., ina PV device, e.g., as part or all of a photoactive layer. Thenon-randomly oriented nanostructures can include both regularly-orderedarrays of nanostructures, as well as irregularly-ordered arrangements ofnanostructures.

In another aspect, the present invention provides compositions having anarray of nanostructures in a matrix, e.g., in a photoactive layer, inwhich the plurality of nanostructure members are non-randomly arranged.Optionally, the nanostructures are non-randomly oriented in addition tobeing non-randomly arranged.

In a further embodiment, the compositions of the present invention havetwo or more matrix layers, each member layer comprising a plurality ofnon-randomly oriented nanostructures and/or non-randomly arrangednanostructures. The member nanostructures in the matrix layers can bealigned with respect to the member nanostructures in an adjacent matrixlayer (e.g., where the photoactive layer or the overall device comprisesmore than one active layer), or not aligned.

In any case, methods of orienting nanorods in a polymer matrix includeelectric field assisted orientation of nanorods, e.g., applying anelectric field to cause magnetic rods to orient in a desired direction,in a polymer matrix which optionally can be hardened to maintain theorientation. Alternatively, rods may be disposed in a polymer matrixwhich can then be stretched to cause the rods to align in the directionof stretching. In particular, stretching of polymer materials aligns thepolymer strands in the direction of stretching which tends to forcesimilar alignment of any nanorods disposed in the polymer material.

Alternatively, aligned rods may be grown substantially in situ as afield of oriented structures that are subsequently integrated into thepolymer matrix and active layer. Methods of fabricating such fields ofaligned nanostructures are described, in e.g., Published U.S. patentapplication Nos. 2002/0172820 and 2002/0130311, the full disclosures ofwhich are hereby incorporated by reference in their entirety for allpurposes.

In alternative preferred aspects, alignment of rods may be achievedthrough the exploitation of liquid crystal phases of the nanocrystals.For example, CdSe nanorods have been observed to exhibit liquidcrystalline phases under certain conditions. Once in these liquidcrystal phases, one can readily align such crystals by applying a smallelectric field as is commonly done with organic liquid crystals, e.g.,in display applications.

Alternatively, self assembly methods may be employed that rely ondifferential treatment of one or both ends of a nanorod, as compared tothe side surfaces. In preferred aspects, the rods would be alignedthrough the incorporation in the matrix of liquid crystals. For example,one could provide a hydrophilic moiety on an end of a nanorod whileproviding hydrophobic sides, in order to force rods to orient along anaqueous-organic interface with the end extending into the aqueous phasewhile the side surfaces remained in an organic phase. Curing of theorganic phase, e.g., a polymer, would then secure the rods in position.Relatedly, one could provide layers of matrix material that havedifferent affinities for other chemical moieties that are selectivelydeposited on different portions of rods or wires. For example, a rod maybe provided with a first chemical moiety at one end, e.g., that portionthat has a wurtzite structure, while a different chemical moiety iscoupled to the other end or the remaining portion of the rod or wire. Bydepositing the rods or wires into a matrix that includes an interfacewhere one side of the interface has a higher affinity for the firstchemical moiety and the other side of the interface has a higheraffinity for the second chemical moiety, it may result in selforientation of the rods across the interface.

In another alternative method, nanorods may be aligned and oriented bycoupling a linker molecule, e.g., an organic surfactant, that bindstrongly to only one end of the nanorods. The other end of thesurfactant is then functionalized to bind selectively to one electrode.By way of example, in the case of CdSe, alkyl phosphonic acids bind morestrongly to the 00-1 face of nanorods than any other face. Thisselective binding is then taken advantage of in coupling the bound endto an electrode, e.g., the cathode, of a photovoltaic device.

In one general class of embodiments, the photoactive layer includescompositions having a plurality of selectively-oriented nanostructuresin which the members nanostructures are associated with one or morealignment ligands (which can be components of a matrix in which thenanostructures are embedded, or can simply be directly attached to thenanostructures). In one example, a first alignment ligand on a firstmember nanostructure interacts with a second alignment ligand associatedwith an adjacent member nanostructure, thereby selectively-orienting theplurality of nanostructures. These ligands can include any thatordinarily interact with one another, e.g., as in avidin-biotin,antibody-antibody ligand, aptamer-aptamer binding moiety, complementarynucleic acids, interactive chemical moieties, and/or the like.

In any case, in one embodiment of the present invention, a plurality ofselectively-oriented nanostructures are provided in the photoactivelayer. The nanostructures are selectively aligned by providing aplurality of nanostructures comprising a first set of nanostructuresassociated with a first alignment ligand and a second set ofnanostructures associated with a second alignment ligand, andinteracting the first alignment ligand on a first nanostructure with thesecond alignment ligand on a second adjacent nanostructure, toselectively orient the plurality of nanostructures.

In another related embodiment, a plurality of non-randomly oriented ornon-randomly dispersed nanostructures in a matrix in the photoactivelayer are prepared by providing a plurality of nanostructures and amatrix composition, in which the matrix composition includes one or morematrix components that interact to form a plurality of receivingstructures capable of accommodating the nanostructures. The compositionis heated and cooled in the presence of the plurality of nanostructures,thereby preparing the plurality of non-randomly oriented or non-randomlydispersed nanostructures in the matrix.

In any case, aligned and/or organized nanostructures can be used in anyof a number of devices and applications, including, but not limited to,various photovoltaic devices, optoelectronic devices (LEDs, nanolasers),light collectors, and the like.

In certain preferred aspects, branched nanorods, and particularly,branched nanorods having a tetrahedral geometry, e.g., nanotetrapods,are used as the electron conducting component of the active layer. Inthe case of nanotetrapods, virtually any orientation will provide aneffective conductive path that is oriented substantially orpredominantly normal to the plane of the electrodes, as described above,and thus no orientation process or step is required. In particular,because nanotetrapods have four branches arranged in a tetrahedralarchitecture, virtually any rotational disposition of the structure willprovide a reasonably direct conductive path through the active layer.For a nanotetrapod, no branch or component of a single nanotetrapodstructure will be more than 60° from normal to the electrode, and atleast one component or branch will be within 30° of normal to the planeof the electrode. Thus, in terms as described for nanorods,nanotetrapods include at least one elongated section that has alongitudinal dimension or axis that is within 30° of normal.

FIG. 3 schematically illustrates a comparison of electron conduction inactive layers that include spherical nanocrystals (FIG. 3A),non-oriented nanorods (FIG. 3B), oriented nanorods (FIG. 3C), andnanotetrapods (FIG. 3D). As shown in FIG. 3A, an active layer 302 thatemploys spherical nanocrystals 304 necessarily requires multiple hoppingincidents (dashed arrows show exemplary potential conduction path) foran electron to conduct from the middle of the active layer 302 to theelectrode 306. Similarly, for nonoriented rods 310, e.g., as shown inFIG. 3B, although electrons are conducted for longer distances withouthopping, where those distances are not properly oriented, conduction toan electrode 306 can again involve multiple hopping events. As can beseen in FIGS. 3C and 3D, however, oriented nanorods 320, or relatedly,nanotetrapods 330, are capable of providing a direct or nearly directconductive path from the active layer to the electrode 306.

Although described in terms of active layers that include eitherspherical nanocrystals, nanorods or nanotetrapods, it will beappreciated that devices of the present invention may include activelayers that are comprised of multiple different types of nanostructures,e.g., nanorods and nanotetrapods, nanorods and spherical or nonelongatednanocrystals, or nonelongated nanocrystals and nanotetrapods, or allthree types of nanocrystals. Similarly, devices may include multipleactive layers sandwiched between multiple electrode layers, where eachactive layer is made up of homogeneous or heterogeneous mixtures ofnanostructure types, and where each layer is the same as or differentfrom at least one other layer.

As noted above, operation of the photovoltaic devices of the inventionrelies upon unidirectional current flow through the active layer, alsoreferred to as “charge separation.” In the nanocomposite based devicesof the invention, unidirectional conduction may be accomplished by anumber of different means. For example, in certain cases, blockinglayers are employed on the respective electrodes to function aselectrical check valves to maintain charge separation. In particular, ablocking layer is provided proximal to the first electrode that blockselectron conduction to the first electrode, while not blocking holeconduction to (or electron conduction from) that first electrode). Incontrast, another different blocking layer is provided proximal to thesecond electrode that permits electron conduction to the secondelectrode but blocks hole conduction to (or electron conduction from)that second electrode. An example of a device architecture that includessuch blocking layers is shown in FIG. 4A. As shown, the device includesthe same components as shown in FIG. 1. In addition, however, disposedadjacent the one electrode 110, is an electron blocking layer 410, e.g.,a layer that is less or not conductive to electrons, but permits thepassage of holes to electrode 110. A hole blocking layer 420 (e.g., thatblocks the passage of holes but permits passage of electrons from theactive layer to the electrode) is then disposed between the active layer102 and the other electrode 108. Although described as having twoblocking layers, in this aspect of the invention, it is possible thatonly one blocking layer might be provided, e.g., just an electronblocking layer on one electrode or just a hole blocking layer on theother electrode. A variety of different types of blocking layers areknown in the art and include, e.g., hole blocking layers, e.g., TiO₂,electron blocking layers, e.g., crosslinkedpoly(3-hexylthiophene)(P3HT), carboxylated P3HT, crosslinked TPA, andthe like. Although described as a blocking layer for holes or electrons,it will be appreciated that this generally refers to relativeconductivity, e.g., a material that is a better hole conductor than itis an electron conductor would be referred to as an electron blockinglayer in accordance with the invention, and vice versa. Because of theapplication of the devices of the invention, blocking layers that arehighly stable to exposure to light, oxygen and water, e.g., TiO₂, may beparticularly preferred.

Alternatively or additionally, one can enhance unidirectional currentflow in the devices of the invention by providing the nanocrystal (orother nanostructure) component positioned proximal to one electrode,versus the other. An example of this type of structure is illustrated inFIG. 4B. In particular, as shown, the active layer 102 includes ananorod component 104 (as shown) and a polymer matrix component 106(hatched) disposed between first and second electrodes 108 and 110,respectively. As shown in FIG. 4B, however, the nanorods 104 arepositioned closer to the lower or first electrode 108 than to secondelectrode 110. By providing the electron conducting nanocrystalcomponent 104 predominantly closer to one electrode 108 than to theother electrode 110, one enhances electron conduction to the firstelectrode 108 over the second electrode 110 which is partially insulatedby the polymer matrix 106. Similarly, by a greater concentration ofpolymer proximal to the second electrode 110, one reduces the potentialfor electron flow to the second electrode from the active layer, e.g.,from nanorods 104. For extra measure, one can also provide one or moreblocking layers in addition to providing for positioned nanostructures,e.g., electron blocking layer 410 and hole blocking layer 420. In thecase of the nanocrystal component, the phrase “predominantly moreproximal to one electrode” than the other electrode means that at least60% of the nanocrystals in the active layer are disposed closer (interms of distance from any portion of the nanocrystal to any portion ofthe electrode) to one electrode than to the other. Preferably, at least80% or even 90% or more of the nanocrystals are closer to one electrodethan to the other.

One method of preferentially locating nanocrystals (or othernanostructures) closer to one electrode than the other is tosequentially deposit the active layer components onto one electrode. Inparticular for example, one can deposit a portion of the active layermatrix that includes the nanocrystals onto the first electrode, e.g.,electrode 108. A second layer of just the matrix material, e.g., theconductive polymer, is then deposited over the portion that contains thenanocrystals. This will result in an increased proportion ofnanocrystals closer to one electrode (108) than the other (110).Alternatively, one could perform a modified version of this type ofoperation in reverse. In particular, one could first deposit a layer ofconducting polymer onto one electrode (110). A second layer ofconducting polymer/nanocrystal matrix, is then deposited over the purepolymer layer, and the other electrode (108) is deposited onto theactive layer. In order to provide direct connection of the nanocrystalsto the other electrode (108), one may optionally or additionally thenpreferentially dissolve the polymer layer away from the matrix, e.g.,dissolving 1%, 10%, 20%, 50% or more of the polymer in the matrix layer,to expose the nanocrystal components of the matrix. The second electrode(108) is then deposited upon the exposed nanocrystals to provide suchcrystals in direct contact with the second electrode, but sufficientlydistant from the first electrode, by virtue of the pure polymer layerthat was first deposited thereon.

In additional or alternative arrangements, one may enhance chargeseparation through the use, as noted previously, of core-shell nanorods,where the core and shell have a type II band offset, e.g., conductingone charge carrier through the core and the other through the shell,thereby functioning in the same way as a nanocrystal-conducting polymercomposite active layer. In such cases, the core-shell nanocrystalfunctions as the active layer component as shown in zones II and III inFIG. 2. By using a core-shell nanocrystal as the active layer, oneobviates the need for the conductive polymer contribution to theoperation of the device, e.g., the shell (or core, depending uponcomposition) functions in the same manner as the polymer, e.g., as thehole carrier. In such cases, one may eliminate the conductive polymerfrom the active layer. In a further modification of the above describedselective etching process for positioning nanocrystals into electricalcontact with one electrode, one could optionally or additionally etchaway a portion of a shell material, in the case where an active layer ofcore/shell nanocrystals are used. The ensuing deposition of theelectrode (108) layer would then provide the core of the nanocrystal,e.g., the electron conducting portion, in direct electrical contact withelectrode 108, further enhancing the efficiency of the electronconductance. Similarly, where the core material is a hole conductor, thereverse operation, e.g., depositing electrode 110 over the exposed core,would be done. Because cores and shells are typically of relativelysubstantially dissimilar materials, one can adopt an etchant compositionand/or process, e.g., timing, concentrations of etchant, etc., thatselectively etches the shell without excessively etching the core.

In a related aspect, an active layer may be fabricated by providingcore-shell nanocrystals as a component of the active layer, e.g., asdescribed above. However, in optional additional aspects, the core-shellnanocrystal component of the active layer is sintered or otherwiseagglomerated or linked together physically and/or electrically. Theresulting active layer is then comprised of an amorphous matrix thatincludes regions of shell material substantially evenly dispersed in thematrix with regions of core material. These regions retain theirfunctionality as one of the hole or electron conductor, in the activelayer. Further, because they originate from nanocrystals, the amorphousmatrix appears to be substantially homogeneous, at the micron scale,while actually being heterogeneous at the nanoscale level. Whileadvantageous from some perspectives, e.g., charge separation andconduction, sintered active layers may be less flexible than polymercomposite active layers, and thus may be preferred for certainapplications and less preferred for others. Sintering is typicallyaccomplished using well known thermal sintering methods whereby thenanocrystal layer is heated to a temperature at which agglomerationoccurs, but is still typically below the melting point of thenanocrystal materials. For such sintered active layers, it may bedesired to provide the nanocrystals as a substantially contiguousmonolayer within the active layer, which is not always necessary forpolymer composite active layers.

B. Inorganic Active Layers

One class of embodiments provides photovoltaic devices in which theactive layer comprises one or more populations of nanostructurescollectively comprising two inorganic materials (e.g., one material thatfunctions as a hole conductor and one that functions as an electronconductor). These photovoltaic devices are similar to those describedabove, although the nanostructures are not necessarily nonrandomlyoriented and/or are not necessarily part of a nanocomppsite. Thus, inone class of embodiments, the photovoltaic device comprises a firstelectrode layer, a second elector layer, and a first photoactive layerdisposed between the first and second electrode layers. The photoactivelayer is disposed in at least partial electrical contact with the firstelectrode along a first plane and in at least partial electrical contactwith the second electrode along a second plane. The photoactive layercomprises a first inorganic material and a second inorganic materialdifferent from the first inorganic material; the first and secondinorganic materials exhibit a type II band offset energy profile. Inaddition, the photoactive layer comprises a first population ofnanostructures, which nanostructures comprise the first inorganicmaterial, the second inorganic material, or a combination thereof.

In a preferred class of embodiments, the first and second inorganicmaterials are semiconductors. Essentially any pair of semiconductormaterials with a type II band offset can be used. For example, the firstinorganic material can comprise a first semiconductor selected from thegroup consisting of: a Group II-VI semiconductor, a Group II-Vsemiconductor, a Group IV semiconductor, and an alloy thereof.Similarly, the second inorganic material can comprise a secondsemiconductor, different from the first semiconductor, selected from thegroup consisting of: a Group II-VI semiconductor, a Group III-Vsemiconductor, a Group IV semiconductor, and an alloy thereof. Examplesemiconductors include, but are not limited to, CdSe, CdTe, InP, InAs,CdS, ZnS, ZnSe, ZnTe, HgTe, GaN, GaP, GaAs, GaSb, InSb, Si, Ge, AlAs,AlSb, PbSe, PbS, and PbTe.

The nanostructures can be any of a variety of nanostructures. Forexample, the nanostructures-can comprise a single-crystal nanostructure,a double-crystal nanostructure, a polycrystalline nanostructure, or anamorphous nanostructure. The nanostructures can comprise, e.g.,nanocrystals, nanorods, nanowires, nanotubes, branched nanocrystals,nanotetrapods, tripods, bipods, nanodots, quantum dots, nanoparticles,branched tetrapods, or the like, or a combination thereof (e.g.,nanorods and nanotetrapods, or nanorods and spherical or nonelongatednanocrystals, among many other possible combinations).

As noted above, an active layer can be fabricated by providingcore-shell nanocrystals. Thus, in one class of embodiments, the firstpopulation of nanostructures comprises nanocrystals that comprise a coreof the first inorganic material and a shell of the second inorganicmaterial. For example, as noted previously, the first and secondinorganic materials can be essentially any two semiconductors with anappropriate type II band offset; for example, the core can comprise CdSeand the shell CdTe, or the core can comprise InP and the shell GaAs.

As noted above, the nanocrystals are optionally fused, partially fused,and/or sintered. In one class of embodiments, the cores of at least twoadjacent nanocrystals are in at least partial direct electrical contactand the shells of at least two adjacent nanocrystals are in at leastpartial direct electrical contact. The cores of one or more nanocrystalsare optionally in at least partial direct electrical contact with thefirst electrode or the second electrode; similarly, the shells of one ormore nanocrystals are optionally in at least partial direct electricalcontact with the opposite electrode.

Operation of an example embodiment in which adjacent core-shellnanocrystals are fused, partially fused, or sintered such that the coresand/or shells of adjacent nanocrystals are in at least partial directelectrical contact is schematically illustrated in FIG. 10. In thisexample, the general device structure 1000 comprises an active layer1002. As shown, the active layer includes a nanocrystal component havinga core 1004 and a shell 1006, the materials of which exhibit a type IIband offset. The active layer 1002 is disposed between first electrode1008 and second electrode 1010 (which is illustrated disposed on aseparate substrate 1016, although the electrode(s) and substrate can bea single integral unit). When light (arrow 1012) passes through thetransparent electrode and substrate in this example, it impinges uponthe nanocrystal, creating an exciton. The hole (Θ) is conducted by theshell material 1006 to electrode 1010, while the electron (e⁻) isconducted through the core material 1004 to electrode 1008. Theresulting current flow in the direction of arrows 1015 is then exploitedin load/device 1014. In this example, the cores of some nanocrystals arein at least partial direct electrical contact with the electrode 1008,while the shells of other nanocrystals are at least partial directelectrical contact with the electrode 1010. As an alternative example,the nanocrystals can comprise a hole conducting core and electronconducting shell, in which example the cores would contact electrode1010 and the shells electrode 1008.

In a related class of embodiments, the photoactive layer comprises twopopulations of nanostructures. The first population of nanostructurescomprises nanocrystals comprising the first inorganic material, and asecond population of nanocrystals comprises nanocrystals which comprisethe second inorganic material. As in the preceding embodiments, thefirst and second materials can be, for example, any two semiconductorswith a type II band offset. For example, the first inorganic materialcan comprise CdSe and the second CdTe, or the first inorganic materialcan comprise CdS and the second CdTe, or the first inorganic materialcan comprise CdS and the second ZnSe. Adjacent nanocrystals in thephotoactive layer are typically in at least partial direct electricalcontact with each other. The nanocrystals of the first and/or secondpopulations are optionally fused, partially fused, and/or sintered (ornot fused, partially fused, and/or sintered).

In one class of embodiments, the nanocrystals of the first populationand the nanocrystals of the second population are intermixed in thephotoactive layer. The nanocrystals of the two populations can beuniformly mixed, or an increased proportion of one population can becloser to one electrode while an increased proportion of the otherpopulation is closer to the other electrode.

Operation of an example embodiment in which the photoactive layercomprises two intermixed nanocrystals populations is schematicallyillustrated in FIG. 11. In this example, the general device structure1100 comprises an active layer 1102. As shown, the active layer includesone population of nanostructures 1106 comprising a hole conductinginorganic material and another population of nanostructures 1104comprising an electron conducting inorganic material, where the twomaterials exhibit a type II band offset. Adjacent nanocrystals are in atleast partial direct electrical contact with each other. The activelayer 1102 is disposed between first electrode 1108 and second electrode1110 (which is illustrated disposed on a separate substrate 1116,although the electrode(s) and substrate can be a single integral unit).When light (arrow 1112) passes through the transparent electrode andsubstrate in this example, it impinges upon the nanocrystal, creating anexciton. The hole (Θ) is conducted by the nanocrystals 1106 to electrode1110, while the electron (e⁻) is conducted through the nanocrystals 1104to electrode 1108. The resulting current flow in the direction of arrows1115 is then exploited in load/device 1114.

In another class of embodiments, the nanocrystals of the two populationsare segregated into layers rather than being intermixed. In theseembodiments, the photoactive layer comprises at least a first sublayerand a second sublayer, wherein the first sublayer comprises the firstpopulation of nanocrystals and the second sublayer comprises the secondpopulation of nanocrystals.

Operation of an example embodiment in which the photoactive layercomprises two sublayers is schematically illustrated in FIG. 12. In thisexample, the general device structure 1200 comprises an active layer1202. As shown, the active layer includes one sublayer comprising onepopulation of nanostructures 1206 and another sublayer comprising apopulation of nanostructures 1204. The nanostructures 1206 comprise ahole conducting inorganic material, while the nanostructures 1204comprise an electron conducting inorganic material, where the twomaterials have a type II band offset. The active layer 1202 is disposedbetween first electrode 1208 and second electrode 1210 (which isillustrated disposed on a separate substrate 1216, although theelectrode(s) and substrate can be a single integral unit). When light(arrow 1212) passes through the transparent electrode and substrate inthis example, it impinges upon the nanocrystal, creating an exciton. Thehole (Θ) is conducted by the nanocrystals 1206 to electrode 1210, whilethe electron (e⁻) is conducted through the nanocrystals 1204 toelectrode 1208. The resulting current flow in the direction of arrows1215 is then exploited in load/device 1214.

As noted, the photoactive layer can comprise at least two activesublayers, e.g., wherein each of the active sublayers comprises aplurality of nanocrystals of at least one nanocrystal type. In one classof embodiments, at least one of the at least two sublayers comprises ann-type sublayer and at least one of the two sublayers comprises a p-typesublayer; optionally, the photoactive layer comprises a junction betweenthe p-type sublayer and the n-type sublayer. At least one of thesublayers is typically nanocrystalline. In another class of embodiments,the photoactive layer comprises a blend of p and n nanocrystals. Forexample, the photoactive layer can comprise at least one sublayercomprising a blend of p and n nanocrystals.

As noted above, use of inorganic nanostructures to provide both hole andelectron conducting materials in the active layer can obviate the needfor a conductive polymer component in the active layer. Thus, althoughin some embodiments the photoactive layer further comprises a conductivepolymer (e.g., a conductive polymer matrix in which the nanostructuresare disposed), in alternative embodiments, the photoactive layer issubstantially free of conductive polymer. In certain embodiments, thephotoactive layer also includes a nonconductive polymer or other binder(e.g., a nonconductive polymer matrix in which the nanostructures aredisposed, uniformly or nonuniformly). It will be appreciated thatpreferred polymers will be light stable, and, depending on the packagingof the devices, oxygen stable. Suitable conductive polymers include,e.g., those noted in the above embodiments; suitable nonconductivepolymers include, e.g., polyamide and PMMA. The photoactive layeroptionally includes components included to modify the properties of thenanostructures, e.g., surface chemistries and ligands such as thosedescribed above.

The nanostructures can be, but are not necessarily, nonrandomly orientedin the photoactive layer. Thus, in one class of embodiments, thenanostructures of the first population (and/or the second population, ifpresent) each has at least one elongated section oriented predominantlynormal to at least the first plane. Such orientation can be achieved,e.g., as described above, e.g., for nanorods. As one example, thenanostructures can comprise branched nanocrystals having more than oneelongated segment, e.g., branched nanocrystals comprising four elongatedsegments connected at a common apex and arranged in a substantiallytetrahedral geometry.

As described above, blocking layers can be employed to maintain chargeseparation. Thus, in one class of embodiments, the photovoltaic devicealso includes a hole or electron blocking layer disposed between thephotoactive layer and the first or second electrode. For example, thephotovoltaic device can include a hole blocking layer disposed betweenthe photoactive layer and the first electrode and an electron blockinglayer disposed between the photoactive layer and the second electrode.

As will be discussed in greater detail below, for certain applications,the photovoltaic device is desired to be flexible or otherwiseconformable. Thus, at least one of the first and second electrodes areoptionally flexible, as is the photoactive layer. The device isoptionally hermetically sealed and can assume any of a variety ofarchitectures.

The composition and/or size of nanocrystals can be selected, e.g., tooptimize the absorption spectrum of the active layer, as discussed ingreater detail in the following section. Thus, the first population ofnanostructures optionally comprises at least two different nanocrystalsubpopulations, each nanocrystal subpopulation having a differentabsorption spectrum. For example, the different nanocrystalsubpopulations can comprise different compositions and/or the differentnanocrystal subpopulations can comprise nanocrystals having differentsize distributions.

The photovoltaic device optionally comprises at least a secondphotoactive layer, e.g., optimized for absorption of a wavelength rangedistinct from that of the first photoactive layer. Thus, in one class ofembodiments, the photovoltaic device further comprises a third electrodelayer, a fourth electrode layer, and a second photoactive layer disposedbetween the third and fourth electrode layers. The second photoactivelayer is disposed in at least partial electrical contact with the thirdelectrode along a third plane and in at least partial electrical contactwith the fourth electrode along a fourth plane. The second photoactivelayer comprises a second population of nanostructures having a differentabsorption spectrum from the first population of nanostructures(typically, a second population of inorganic nanocrystals, similar tothat described for the first photoactive layer). The third electrodelayer, fourth electrode layer and second photoactive layer are attachedto, but electrically insulated from, the first electrode layer, secondelectrode layer and first photoactive layer.

C. Tuning Nanostructure Absorption

In addition to the physical positioning and orientation of thenanostructure (e.g., nanocrystal) component of the active layer, one canadjust or tune the absorption spectrum of the active layer or layers byadjusting the composition of the nanostructure (e.g., nanocrystal)component or components to fit the needs of the particular application.In particular, as noted previously, the absorption spectrum ofsemiconductor nanocrystals can be adjusted depending upon thecomposition and/or size of the nanocrystals. For example, InAs nanorodshave a greater absorption in the near IR range, e.g., the absorption isred shifted as compared to other nanorods, InP nanorods have a greaterabsorption in the visible range, CdSe rods have greater absorption inthe visible to blue range, while CdS nanorods have an absorptionspectrum that is further blue shifted than CdSe nanorods. A chart ofabsorption spectra of CdSe nanorod compositions having three differentsize distributions, e.g., diameters, is shown in FIG. 5. In particular,absorption spectra for the CdSe nanorods become more red-shifted as theyincrease in diameter. As can be seen from the figure and as describedherein, by selecting nanorod composition and size, one can select theabsorption spectrum of the active layer. Further, by combining nanorodsof different size and/or composition, one can tailor a more broadabsorption spectrum for the active layer, in order to optimize thefunctioning of the device over a wider range of conditions.

By way of example, where one wishes to produce a device useful inabsorbing solar energy, one can select nanocrystals or combinations ofnanocrystals for the active layer(s) whose absorption spectra overlap tomore directly match that of the sun in order to more efficiently convertmore of the solar (or other light source) spectrum. Such heterogeneousselections of nanocrystals may be combined as a mixture in a singleactive layer, or they may be provided in multiple discrete layers in amulti-layered device. In some preferred aspects, such differentcollections of nanocrystal sizes and compositions are provided indiscrete layers, so as to maximize the energy conversion from a portionof the relevant spectrum, e.g., each layer includes sufficientconcentrations of nanocrystals of a given absorption wavelength range toyield optimal conversion of light in that range.

II. Electrodes

As described previously, the active layer is sandwiched between twoconductive layers that operate as electrodes for the photovoltaicdevice. In its simplest form, e.g., as shown in FIG. 1, the electrodesimply comprises a first conductive layer upon which is coated orotherwise deposited the active layer, so that the first conductive layeris electrically connected to the active layer. A second conductive layeris then placed on top and in electrical communication with the activelayer. In operation however, a number of considerations must be taken inselecting the appropriate material from which to produce the variousconductive layers. As noted elsewhere herein, the coupling of the activelayer to the electrodes will typically be such that either electrons orholes are passed from the active layer to the particular electrode,e.g., as a result of selection of electrode materials having higher orlower work functions than the active layer, through the incorporation ofblocking layers, etc. As used herein, the phrase “in electricalcontact,” “in electrical communication,” “electrically coupled to,”“electrically connected to,” etc., encompasses such connection whetherit be bidirectional electrical current flow or unidirectional electricalcurrent flow, or any hybrid, e.g., uneven bidirectional current flow.

In the photovoltaic devices described herein, the electrodes that boundthe active layer are typically provided to further enhanceunidirectional current flow through the overall device. In particular,opposing electrodes are typically provided to have different workfunctions, so as to permit the flow of electrons from the active layerinto one electrode, while permitting the flow of holes from the activelayer to the other electrode (or electrons from the other electrode intothe active layer). Typically, this is carried out by providing each ofthe electrodes being fabricated from different materials. For example,by providing one ITO electrode and one aluminum electrode, one canprovide for flow of holes into the ITO electrode and the flow ofelectrons into the aluminum electrode. A variety of different materialsmay be employed for the electrodes, provided they generally meet thesecriteria. Further, electrode material selection will depend, in part,upon the architecture of the device, e.g., as described herein. Forexample, in some cases, transparent conductive materials, e.g., ITOlayered on a transparent substrate, will be desired for at least oneelectrode, to provide light access to the active layer. In otherembodiments, opaque electrode materials are easily employed.

In a number of applications, at least one electrode covers the exposedsurface of the device, e.g., the portion of the device exposed to solaror other light energy. In such cases, it is necessary that the electrodeat this surface be provided as a transparent or translucent layer.Conventional photovoltaic devices typically employ a glass basedelectrode upon which is provided a conductive coating, such as anIndium-Tin oxide (ITO) layer. The glass and ITO layer are transparent,allowing light to pass through the electrode and impinge on the activelayer. Such electrode configurations are readily employed in thenanocomposite and other photovoltaic and layered devices describedherein.

However, for a number of applications, the ultimate device is desired tobe flexible or otherwise conformable in a way that use of glass layersmay not be practical or even possible. In such cases, it will generallybe desired that the electrodes used be flexible as well. Where typicalsandwich style device architecture is employed, a transparent polymericmaterial that employs a metallic or other conductive coating may be usedas the electrode layer. Examples of such polymeric materials include,e.g., polyester films or sheeting, e.g., Mylar films available from,e.g., DuPont, polycarbonate films, polyacetate films, polystyrene filmsor the like. Polymer materials are generally widely commerciallyavailable from a variety of sources. In certain preferred aspects, athin glass layer is used in place of or in addition to a polymer layerto prevent oxygen permeability of many polymer films. Such thin glasssheeting, e.g., on the order of 50 μm thickness or less, can retain theflexibility of most polymer sheets. In such cases, an additional polymercoating may be desirable to prevent scratching of the glass sheet whichcan lead to breakage. Such polymer layers may be made conductive, asnoted previously, by providing a conductive coating, i.e., ITO,aluminization, gold, PEDOT, or other thin, e.g., evaporated, conductivemetal films.

In some cases, it will be desirable to protect the active layer fromexcessive or any exposure to oxygen. In particular, a number ofconductive polymers described above are relatively unstable to oxygen,and lose their conductivity upon exposure. As such, it will often bedesirable to appropriately seal the device to prevent such exposure. Insome cases, the sealing operation may be accomplished through theappropriate selection of electrode layer material. For example, byproviding an oxygen impermeable electrode layer on both sides of theactive layer, one can protect that layer from oxygen exposure. A numberof different polymeric films may be used to provide the oxygen barrier,and yet still provide for flexibility and transparency. Similarly, suchfilms may be readily coated or otherwise treated to render the filmconductive, so that it may function as one of the electrodes of thedevice. In at least one preferred example, an aluminized polymer film isprovided as the electrode layer. By employing an oxygen getter, e.g.,aluminum, magnesium, etc., as the conductive coating on a flexibletransparent layer, one can further reduce the potential for oxygenexposure.

Alternatively, the sealing function may be provided by an additionallayer that is added separate from the electrode layer. This may be thecase for a traditional planar architectures, e.g., non-flexible, oralternatively, for one of the alternate architectures which arediscussed in greater detail below. Regardless of the nature of thesealing layer as a portion of the electrode or as a separate layer, itwill be appreciated that the sealing function must encompass the entireactive layer. As such, the electrode layer may include nonconductiveportions that extend beyond the edge of the conductive portion and theactive layer, and which provide the region that seals against acorresponding region of the opposing electrode layer, e.g., using aheat-melt or adhesive-based sealing process. An example of this type ofassembly is shown in FIG. 6. As shown, the device includes an activelayer 602 sandwiched between two sealing sheets, e.g., film 604 and 606.As noted above, these sheets may be integrated with the electrodes, orthey may be separate layers from one or both of the electrodes. Wherethese sheets are integrated with the electrode components, e.g., sheetelectrodes, it will be appreciated that the conductive portion, e.g.,608, of the electrodes or sealing sheets does not extend to overlappingregions 610, where the sealing sheets are bonded together to encase andseal the device and thus protect the active layer. The sealing functionis illustrated using alternate means in FIG. 6. As shown in the upperleft panel, overlapping sealing sheets are sealed together directly,e.g., thermally bonded at the overlapping regions. Alternatively, asshown in the lower panel, an intermediate layer 612, e.g., an adhesive,or other bonded layer is provided between the sealing sheets, but whichdoes not detract from the sealing function. A finished device 600 isalso shown that schematically illustrates the architecture of an overalldevice, including electrical connections 614 and 616.

For other architectures, described in greater detail below, it is notnecessary to provide the electrodes as transparent layers, e.g., wherelight need not pass through such electrodes. In such cases, any of theaforementioned flexible or other electrode configurations can be used.Alternatively, thin, flexible, metal foil electrode layers may be used.Further, in such cases, any sealing layer may not be layered onto theelectrode layer, but may exist in a completely separate plane.

Further, in the case of alternative architectures, the function ofprotecting the polymer matrix from oxygen, where necessary, may beprovided by the packaging of the device itself. For example, a flexible,transparent envelope may be provided to seal the device and preventoxygen exposure of the active layer. Alternatively, where flexibility ofthe packaged device is not critical, these alternate architecturedevices could be housed in more conventional packages, e.g.,hermetically sealed behind a glass or other oxygen impermeable barrier.By way of example, and as discussed in greater detail below, sideexposed photovoltaic devices may be encased in a transparent orpartially transparent package, where one of the sealing layers isdisposed over a side edge of the electrode/active layer/electrodeportion of the device.

Although described as planar or sheet type electrodes, it will beappreciated that wire electrodes may also be employed in the devicesdescribed herein. Such wire electrodes may be an adjunct to one or moresheet or planar electrodes, or they may be in place of such planarelectrodes, e.g., integrated into the active layer. By way of example,wire electrodes may be provided within or on one and/or the othersurface of the active layer. The wires may be coated with blockinglayers depending upon the function they are to serve, e.g., they may becoated with an electron blocking layer or a hole blocking layer. Incertain cases, the wires may be integrated into the active layer, e.g.,disposed within the interior of the active layer, e.g., as overlappingarrays of wires or interspersed with complementary wires, e.g., ofalternate blocking layer coatings. The device may include bothelectrodes as wires or wire arrays, or may include one wire arrayelectrode and one sheet electrode.

III. Device Architecture

Again, as noted above, the device architecture in its simplest formcomprises a basic planar sandwich structure, e.g., as shown in FIG. 1.However, because of the various improvements described herein, a numberof other architectures may be employed that provide for advantages in anumber of different applications. These various architectures aregenerally made possible by the flexible or conformable nature of thematerials used to fabricate the devices described herein, propertiesthat are lacking in materials used in conventional, rigid photovoltaics.

As noted previously, the basic architecture of a photovoltaic device inaccordance with the invention comprises an active layer that is disposedbetween two electrodes. The active layer, as termed herein, generallyrefers to that portion or layer of the device in which light inducedcharge separation occurs. By light induced charge separation, is meantthe generation of free electrons or electron/hole pairs, as a result ofphotons impinging on the active layer. Such light induction may bedirect, e.g., light impacting a material results in the liberation of anelectron, or it may be indirect, e.g., light impacting the active layerresults in a chemical, physical or electrical change that later resultsin the liberation of an electron, e.g., through a chemical or physicalpathway.

Through the use of the nanocomposite active layers or certain of theother nanostructure based active layers and certain of the electrodeconfigurations described herein, one can produce photovoltaic devicesthat are efficient, flexible and lightweight. With thesecharacteristics, it is possible to provide a wide variety of alternativearchitectures for photovoltaic devices and systems that can address manyof the shortcomings of conventional photovoltaic technology.

In a first and likely preferred aspect, the basic photovoltaic device inaccordance with the invention comprises a standard planar sandwichformat, e.g., as shown in FIG. 1. In particular, the overall device 100is configured in a planar format with the active layer 102 sandwichedbetween first electrode layer 104 and second electrode layer 106. Wherethe maximal surface area of the active layer is covered by theelectrodes. As it is generally desirable to expose a larger area of theactive layer to light, e.g., to collect the greatest amount of solarpower, it is generally desirable that such planar devices include atleast one electrode layer that is transparent to light of theappropriate wavelength or translucent. The surface of the active layerthat is exposed to light, whether through an electrode layer orotherwise, is termed the exposed surface. Typically, photovoltaicdevices accomplish this through the use of a glass layer that is coatedwith a transparent conductive layer, e.g., indium tin oxide (ITO) layer,which together function as the electrode. In such cases, a reflectivesurface is generally provided upon the second electrode to maximize theamount of solar light collected by the active layer. Typically, thesereflective electrode layers may comprise any highly conductive metalthat is also relatively highly reflective, e.g., platinum chrome, or thelike.

In alternative aspects, it may be less desirable to provide oneelectrode layer as a transparent layer, e.g., because of inefficienciesin such layers from either a conductivity, flexibility or transmissivitystandpoint. In such cases, alternative architectures may be employedthat allow one to employ different electrode materials. For example, ina first aspect, one of the electrodes may be provided as a flexiblemetal layer, e.g., a metallized polymer sheet, e.g., aluminized Mylar,but including an array of transparent zones, e.g., as a screen. In suchcases, one would lose some of the exposed surface area of the activelayer in exchange for the ability to provide a more efficient firstelectrode layer. An example of a device employing this architecture isillustrated in FIG. 7. As shown, the active layer 702 is layered onto alower electrode 704, and an upper electrode 706 is provided over theactive layer to create the same sandwich architecture described above.Instead of upper electrode 706 being transparent, however, it isprovided with openings or transparent regions 708 which allow light topass through. The use of these transparent regions or openings allowsupper electrode 706 to be comprised of any of a variety of conventionalconductive materials, e.g., metal layers, etc. Further, by using thinfoils of these conductive layers, one can maintain the flexibility ofthe overall device. Typically, a further sealing layer of transparentmaterial, e.g., layer 710, may be provided over the top of the upperelectrode, and optionally, although not shown, on the outer surface oflower electrode 704. The openings 708, although shown as circularopenings, may comprise perforations in a metal layer or the spacebetween gridded arrays of wires, e.g., screen mesh.

In a further alternative architecture, the active layer is disposedbetween two electrode layers, but the exposed surface is a side surface,rather than a surface that is covered by an electrode. A simplifiedexample of this is shown in FIG. 8A. As shown, active layer 802 is againdisposed between two electrode layers 804 and 806. The side or edgesurface 808 of the active layer 802 operates as the exposed surface,e.g., the surface through which light is collected. Typically, atransparent or translucent protective layer (e.g., layer 812 in FIG. 8B)is disposed over this side surface 808, in order to prevent degradationor other damage to the active layer 802. In order to optimize theexposed surface 808 for active layer 802, which in turn, maximizes theamount of light that impinges on the active layer, one can configure theside exposed device in a number of different ways. For example, theoverall device may be fabricated as an elongated laminate film or tape,e.g., as substantially shown in FIG. 8A. Once produced as a flexiblefilm or tape, the overall device 800 may then be coiled or spooled (asshown), in order to provide an array of exposed surfaces. Alternatively,the overall device may be folded, or stacked in a reciprocating orserpentine architecture, e.g., successively folded back and forth uponitself, to provide greater exposed surface area 808, as shown in FIG.8B, or to provide a selected footprint for a particular application.

The use of flexible active layer components, or overall devices allowsfor other useful adjustments in device architecture. By way of example,one of the limits on the output of a photovoltaic device is the amountof surface area that is may be impacted by photons from the relevantlight source, e.g., the sun. In conventional, planar photovoltaics, thenumber of photons that impinges upon the active layer is directlyrelated to the surface area of the device that is exposed to the lightsource. In many applications, the allowable surface area of a device isdictated by the space in which the device must fit, e.g., a roof top,airplane wing, extraterrestrial platform, i.e., satellite, spacestation, etc. By providing a device architecture that includes a convexexposed surface and active layer, e.g., as enabled by the flexibledevices described herein, one can effectively increase the effectivesurface area of the device without increasing its footprint. This isparticularly true where the relevant light source is oriented at asub-normal angle to the device.

FIG. 8C illustrates the benefits of a convex architecture that achievesthis goal. As shown, the device 850 has a convex shape to the exposedsurface 852 and active layer 854. When the light source is directed atthe device from a non-normal angle, e.g., as illustrated with lightsource 856 or 858, the number of photons that impact the exposed surface852 of the device is equivalent to a planar device that has a muchlarger footprint, e.g., footprint 860 or 862, as indicated by the dashedlines. However, because of its convex architecture, the device 850 has amuch smaller footprint, e.g., footprint 864. As noted, where usefulspace is limited, such devices can meaningfully enhance the ability toharness light energy.

In addition to issues of increasing output without increasing footprint,one of the efficiency issues associated with conventional, planarphotovoltaics is that they must be continually rotated, or repositionedin order to provide optimal exposure opportunities, e.g., pointingtoward the sun or other light source. For stationary devices, the resultis that for a substantial period, the exposed surface will be far fromoptimally positioned or oriented.

Again, this issue is addressed by the flexible active layer materialsthat permit the use of contoured exposed surfaces. These contouredexposed surfaces will allow for an optimization of exposure for a widervariety of solar source positions. By way of example, a device may beprovided with a convex or concave architecture, e.g., as shown in FIG.8C. Because of the convex/concave architecture of the device, andparticularly the active layer and exposed surface, one can increase thenumber of photons that impinge on the active layer of the device,regardless of the solar position. As shown in FIG. 8C, use of a convexphotovoltaic device provides the ability to increase the number ofphotons striking a device when the light source is directed at thedevice at an angle, e.g., when the sun is in declination or otherwise ata nonoptimal angle, without increasing its footprint. In particular, asshown, when a light source 856 or 858 shines onto the effective surface852 of the convex device 850, a certain number of photons impinge on theactive layer 854 within the device. Where the sun is directed at theoverall device from an angle, e.g., as shown for the light source 856and 858, the number of photons captured by the convex device isequivalent to that of a conventional planar device having a much largerfootprint 860. In particular, as is shown by the dashed lines, thephotons blocked and thus captured by the device is equivalent to thatwhich would have been captured by a much larger flat device under thesame circumstances. In many cases, the allowable space for aphotovoltaic device or system will be limited, and thus, the convexarchitecture would be highly desirable. As will be appreciated, due tothe non-uniform exposure of the overall effective surface of a convexdevice, preferred such devices will typically segment the overall deviceinto discrete electrical units, or devices, to prevent current generatedin high exposure or light regions from shorting through low exposure ordark regions. Such segmenting may take the form of fabricating discretepouches sealed from one another, where each pouch is a separatelyfunctioning photovoltaic device, e.g., as shown in FIG. 6, but wheremultiple pouches are structurally coupled together, e.g., in a sheet ofmany pouches (not shown).

While described in terms of solar exposure, it will be appreciated thatthe devices described herein, and the various architectures are equallyuseful in other applications where one may wish to alter the angle atwhich light impinges on the device, e.g., when using alternate lightsources to generate power, e.g., lasers, or for use as optical sensors,etc.

IV. Nanostructure Based Compositions

Another aspect of the present invention provides nanostructure basedcompositions, e.g., compositions such as those used in the active layersdescribed above. Thus, one general class of embodiments provides acomposition comprising a first population of nanostructures and a secondpopulation of nanostructures, which first population comprisesnanostructures comprising a first material, and which second populationcomprises nanostructures comprising a second material different from thefirst material. Adjacent nanostructures are optionally in at leastpartial direct electrical contact with each other.

The nanostructures can be any of a variety of nanostructures. Forexample, the nanostructures can comprise a single-crystal nanostructure,a double-crystal nanostructure, a polycrystalline nanostructure, or anamorphous nanostructure. The nanostructures can comprise, e.g.,nanocrystals, nanorods, nanowires, nanotubes, branched nanocrystals,nanotetrapods, tripods, bipods, nanodots, quantum dots, nanoparticles,branched tetrapods, or the like, or a combination thereof (e.g.,nanorods and nanotetrapods, or nanorods and spherical or nonelongatednanocrystals, among many other possible combinations).

In a preferred class of embodiments, the first material is a firstinorganic material and the second material is a second inorganicmaterial. In one class of embodiments, the first material comprises afirst semiconductor and the second material comprises a secondsemiconductor. For example, the first material can comprise an n-typesemiconductor and the second material can comprise a p-typesemiconductor.

In one class of embodiments, the first and second materials exhibit atype II band offset energy profile. Such compositions can, for example,be used as active layers in photovoltaic devices, or in other electronicand optoelectronic devices. In another class of embodiments, the firstand second materials exhibit a type I band offset energy profile. Suchcompositions can, for example, be used in LEDs, or in other electronicand optoelectronic devices (e.g., where charge recombination and photonemission rather than charge separation is desired; as is known in theart, nanostructure emission spectra can be adjusted by controlling thecomposition and/or size of the nanostructures).

Essentially any pair of semiconductor materials with an appropriate bandoffset (i.e., type I or type II) can be used. For example, the firstmaterial can comprise a first semiconductor selected from the groupconsisting of: a Group II-VI semiconductor, a Group III-V semiconductor,a Group IV semiconductor, and an alloy thereof. Similarly, the secondmaterial can comprise a second semiconductor, different from the firstsemiconductor, selected from the group consisting of: a Group II-VIsemiconductor, a Group III-V semiconductor, a Group IV semiconductor,and an alloy thereof. Example semiconductors include, but are notlimited to, CdSe, CdTe, InP, InAs, CdS, ZnS, ZnSe, ZnTe, HgTe, GaN, GaP,GaAs, GaSb, InSb, Si, Ge, AlAs, AlSb, PbSe, PbS, and PbTe.

In one class of embodiments, the nanostructures of the first populationand the nanostructures of the second population are intermixed in thecomposition. The nanostructures of the two populations can be uniformlymixed, or they can be partially or completely segregated, e.g., into twoor more distinct regions, zones, layers, or the like.

The composition can, if desired, be formed into a film, which isoptionally disposed between two electrode layers, e.g., for used in aphotovoltaic device, LED, or other device. In some embodiments, thenanostructures of the two populations are intermixed in the film. Inother embodiments, however, the film comprises at least a first sublayerand a second sublayer; the first sublayer comprises the first populationof nanostructures and the second sublayer comprises the secondpopulation of nanostructures. For example, the first sublayer cancomprise p-type semiconductor nanostructures and the second sublayern-type semiconductor nanostructures, optionally with a p-n junctionbetween the sublayers.

As in the preceding aspects, the nanostructures of the first and/orsecond populations can, but need not be, fused, partially fused, and/orsintered.

In some embodiments, the composition also includes a conductive polymer(e.g., a conductive polymer matrix in which the nanostructures aredisposed. In alternative embodiments (e.g., where the first and secondpopulation of nanostructures collectively comprise both hole andelectron conducting materials), the composition is substantially free ofconductive polymer. In certain embodiments, the composition alsoincludes a nonconductive polymer or other binder (e.g., a nonconductivepolymer matrix in which the nanostructures are disposed, uniformly ornonuniformly). The composition optionally includes components includedto modify the properties of the nanostructures, e.g., surfacechemistries and ligands such as those described above.

The nanostructures of the first and/or second populations can berandomly or nonrandomly oriented. For example, each nanostructure of thefirst and/or second populations can have at least one elongated sectionoriented predominantly normal to a first plane (e.g., a surface of afilm comprising the composition, or a surface of an electrode); asanother example, each nanostructure of the first and/or secondpopulations can have at least one elongated section orientedpredominantly parallel to the first plane.

V. Device Manufacturing

The devices and compositions of the present invention also providesignificant advantages in terms of manufacturability. In particular,current photovoltaic devices rely upon relatively low volume, high costmanufacturing processes in converting wafer scale semiconductormaterials to operable photovoltaic devices. Conventional semiconductorbased photovoltaics are typically manufactured in a batch mode with thenumber of devices being determined by the number of wafers used. Suchmanufacturing processes, because of the starting materials andprocesses, can be very expensive, and are also not easily scaled up tolarge scale manufacturing.

The devices of the present invention, on the other hand, are comprisedof materials that are readily and cheaply available, or can be producedin substantial quantities at relatively low costs. Further, because ofthe nature of the various components of the devices, e.g., the activelayer components and sealing and/or electrode layers, the processes formaking these devices readily lends itself to ultra-high throughputmanufacturing. By way of example, because the devices of the inventionare typically in the form of flexible laminate structures, e.g.,flexible electrodes laminated together with an active layer in betweenthem, they may be produced using high volume film processing techniques,e.g., roll to roll processes, that are conventionally used in the filmmanufacturing and industrial laminate industries, e.g., in makingreflective films, photographic films, etc.

An example of a roll-to-roll process and system 900 for fabricatingdevices in accordance with the invention is shown in FIG. 9. As shown, asource of first substrate material, e.g., a roll of substrate material902, e.g., an aluminized polymer sheet or ITO coated polymer sheet, isrolled out and subjected to a number of deposition steps for the variouslayers of the device. Other methods of providing the substrate sheet,e.g., as an accordion folded sheet, etc. could be used. Typically, thesubstrate layer is passed along a conveyor system, e.g., conveyor belt920. Although described as a conveyor belt, any conveyor system that iscapable of moving sheets of substrates, or continuous sheets could beused, e.g., robot arms, moving platforms, etc. Spin coating systems alsocould be used for smaller sized substrates, and such systems would beencompassed by a conveyor system, as that phrase is used herein. Thevarious layers of material are deposited on the substrate in order toprovide the multilayered devices described herein. For example, a sourceof the nanocomposite photoactive matrix might be provided as a hopper orliquid tank that is fluidly coupled to a deposition system for providingthe layer. Such deposition systems might include spraying nozzles,printing heads, screen printing apparatuses, spreading blades, i.e.,doctor blades, sheer coating systems, or other useful systems fordepositing even, thin films of material, including, e.g., dispensingsystems over spin coaters, tape casting systems, film casting systems,and dip coating systems.

In a particular preferred example, using conventional ink-jet or screenprinting technologies, e.g., as indicated by screen printer(s) 904, onecan begin successively laying down multiple layers of material that formthe photovoltaic device, e.g., layers 906, 908, 910, 912 and 914. Forexample, and as shown, in a first printing step, a first layer ofblocking material 906 is printed onto moving substrate 902. Suchblocking material may be a hole blocking layer or an electron blockinglayer depending upon the resulting orientation of the device. Next, alayer of the photoactive material or composite 908, e.g., thenanocrystal/polymer matrix (or the core shell nanocrystal material) isdeposited onto the blocking layer. In the case where a mechanical orelectrical orientation step is desired for the nanocrystals (or othernanostructures), that would generally be carried out followingdeposition of this layer, but prior to the application of the nextlayer. Heating, drying and/or curing steps may be interjected betweenthe various layering steps, to ensure clean interfaces between thelayers, e.g., minimal mixing, among layers. Next, a second blockinglayer 910 is applied to the upper surface of the active layer. Thisblocking layer will be the complement of the first blocking layer 906,e.g., if the first layer 906 was a hole blocking layer, this layer wouldbe an electron blocking layer. A second sheet of substrate material 914,e.g., a transparent conductive electrode sheet, e.g., ITO coated polymersheet as discussed above, is laminated to the top of second blockinglayer. Alternatively, and as shown, an ITO or other conductive layer maybe applied as a coating layer 912 over the second blocking layer 910,followed by application of the substrate layer 914 which may form thesealing layer. Further, additional layers, e.g., sealing layers or otherelectrode, blocking and active layers may be applied in additionalsteps, e.g., in the case of tandem or multiple active layer devices.

Subsequent process steps then attach electrical connections for thesheets of desired size dimensions.

Thus, one class of embodiments provides methods of producing aphotovoltaic device. In the methods, a first planar substrate having afirst conductive layer disposed thereon is provided. The first substrateis coated with a photoactive matrix that exhibits a type II band offsetenergy profile and that comprises at least a first population ofelongated semiconductor nanostructures, the nanostructures comprising alongitudinal axis, to provide a photoactive layer. The semiconductornanostructures are oriented such that their longitudinal axes arepredominantly oriented normal to the first planar substrate. A secondconductive layer is laminated onto the photoactive layer.

The methods can also include providing a blocking layer on the firstsubstrate prior to coating the first substrate with a photoactive matrixand/or providing a blocking layer on the photoactive layer prior tolaminating the second conductive layer onto the photoactive layer. Themethods optionally include providing one or more sealing layers overopposing surfaces of the photovoltaic device in addition to the firstsubstrate and second conductive layer, whereby the one or more sealinglayers hermetically seal the photovoltaic device. As noted above,heating, drying, and/or curing steps may be inserted between the variouslayering steps, if desired. In addition, the various composition anddevice components noted above can be adapted for used in these methods,as appropriate. For example, the nanostructures can comprise ananocrystal, a nanowire, a single-crystal nanostructure, adouble-crystal nanostructure, a polycrystalline nanostructure, and/or anamorphous nanostructure.

A related class of embodiments provides a system for fabricating aphotovoltaic device. The system comprises a source of a first substratelayer having a first conductive surface, a conveyor system for conveyingthe first substrate layer, and a source of a photoactive matrix fluidlycoupled to a layer deposition system. The layer deposition system is atleast partially disposed over the substrate conveyor system, to providea layer of photoactive matrix on the first substrate layer. The systemalso includes a source of a second conductive material coupled to thelayer deposition system positioned over the substrate conveyor systemfor depositing a layer of the second conductive material onto a layer ofphotoactive matrix deposited on the first substrate layer.

In one class of embodiments, the source of first substrate materialcomprises a rolled sheet of first substrate material. The source offirst substrate material optionally also includes a source of firstconductive material and a deposition system for depositing the firstconductive material onto the first substrate material to provide thefirst conductive surface. Examples of suitable layer deposition systemsinclude, but are not limited to, a doctor-blade, a screen printingsystem, an ink-jet printing system, a dip coating system, a sheercoating system, a tape casting system, and a film casting system. Any ofthe above embodiments can be applied to this embodiment as well, to theextent they are relevant.

Another general class of embodiments provides methods of producing aphotovoltaic device. In these methods, a first planar substrate having afirst conductive layer disposed thereon is provided. To provide aphotoactive layer, the first substrate is coated with a composition thatcomprises a population of nanostructures. The nanostructures comprise acore of a first material and a shell of a second material different fromthe first material. The nanostructures are fused, partially fused,and/or sintered, and a second conductive layer is layered onto thephotoactive layer.

All of the various optional configurations and features noted in theembodiments above apply here as well, to the extent they are relevant;e.g., for types of nanostructures, types of materials, provision ofblocking and/or sealing layers, and the like.

Yet another class of embodiments provides methods of producing a layereddevice comprising two (or more) populations of nanostructures (e.g., aphotovoltaic device, an LED, or the like). The layered device comprisesa first population of nanostructures comprising a first material and asecond population of nanostructures comprising a second materialdifferent from the first material. In the methods, a first substrate isprovided and coated with a composition comprising the first populationof nanostructures to provide a first layer.

In one class of embodiments, the nanostructures of the first and secondpopulations are intermixed in the first layer. In this class ofembodiments, the first substrate is coated with a composition comprisinga mixture of the first and second populations of nanostructures toprovide the first layer.

In another class of embodiments, the layered device comprises at least afirst layer comprising the first population of nanostructures and asecond layer comprising the second population of nanostructures. In thisclass of embodiments, the methods include coating the first substratewith a composition comprising the second population of nanostructures,to provide the second layer. In a related class of embodiments, thesecond population of nanostructures is disposed on the first substrate.For example, nanostructures that cannot be solution-processed (e.g.,some nanowires) can be grown on the first substrate (e.g., on anelectrode); a solvent process can be used to add a layer of solventdispersible nanostructures, as above, or other layers can be laminatedover the first substrate-second nanostructure population layer.

The methods optionally include layering a second conductive layer ontothe first layer (or the second layer, depending on the number andorientation of the layers). Similarly, the methods optionally includeproviding a blocking layer on the first substrate prior to coating thefirst substrate with the composition comprising the first population ofnanostructures and/or providing a blocking layer on the first (orsecond) layer prior to laminating the second conductive layer onto thefirst (or second) layer.

All of the various optional configurations and features noted in theembodiments above apply here as well, to the extent they are relevant;e.g., for types of nanostructures, types of materials, fusing, partialfusing, and/or sintering of nanostructures, and the like.

A related class of embodiments provides a system for fabricating alayered device, where the device comprises a layer in which a first anda second population of nanostructures are intermixed. The nanostructuresof the first population comprise a first material, and thenanostructures of the second population comprise a second materialdifferent from the first material. The system comprises a source of afirst substrate layer, a conveyor system for conveying the firstsubstrate layer, and a source of a composition comprising the first andsecond populations of nanostructures, fluidly coupled to a layerdeposition system. The layer deposition system is at least partiallydisposed over the substrate conveyor system, to provide a layer in whichthe nanostructures of the first and second populations are intermixed onthe first substrate layer.

All of the various optional configurations and features noted in theembodiments above apply here as well, to the extent they are relevant;e.g., for types of nanostructures, types of materials, provision ofblocking and/or sealing layers, fusing, partial fusing, and/or sinteringof nanostructures, provision of a layer of a second conductive material,types of layer deposition systems, and the like. It is worth noting thatthe first substrate layer optionally has a first conductive surface (oris otherwise conductive).

Another related class of embodiments provides a system for fabricating alayered device, where the device comprises a first layer comprising afirst population of nanostructures and a second layer comprising asecond population of nanostructures. The nanostructures of the firstpopulation comprise a first material, and the nanostructures of thesecond population comprise a second material different from the firstmaterial. The system comprises a source of a first substrate layer, aconveyor system for conveying the first substrate layer, a source of afirst composition comprising the first population of nanostructuresfluidly coupled to a layer deposition system (the layer depositionsystem being at least partially disposed over the substrate conveyorsystem, to provide a first layer), and a source of a second compositioncomprising the second population of nanostructures fluidly coupled tothe layer deposition system (the layer deposition system being at leastpartially disposed over the substrate conveyor system, to provide asecond layer).

All of the various optional configurations and features noted in theembodiments above apply here as well, to the extent they are relevant;e.g., for types of nanostructures, types of materials, provision ofblocking and/or sealing layers, fusing, partial fusing, and/or sinteringof nanostructures, provision of a layer of a second conductive material,types of layer deposition systems, and the like.

VI. Synthesis of Nanostructures

Nanostructures can be fabricated and their size can be controlled by anyof a number of convenient methods that can be adapted to differentmaterials. For example, synthesis of nanocrystals of various compositionis described in, e.g., Peng et al. (2000) “Shape control of CdSenanocrystals” Nature 404, 59-61; Puntes et al. (2001) “Colloidalnanocrystal shape and size control: The case of cobalt” Science 291,2115-2117; U.S. Pat. No. 6,306,736 to Alivisatos et al. (Oct. 23, 2001)entitled “Process for forming shaped group III-V semiconductornanocrystals, and product formed using process”; U.S. Pat. No. 6,225,198to Alivisatos et al. (May 1, 2001) entitled “Process for forming shapedgroup II-VI semiconductor nanocrystals, and product formed usingprocess”; U.S. Pat. No. 5,505,928 to Alivisatos et al. (Apr. 9, 1996)entitled “Preparation of III-V semiconductor nanocrystals”; U.S. Pat.No. 5,751,018 to Alivisatos et al. (May 12, 1998) entitled“Semiconductor nanocrystals covalently bound to solid inorganic surfacesusing self-assembled monolayers”; U.S. Pat. No. 6,048,616 to Gallagheret al. (Apr. 11, 2000) entitled “Encapsulated quantum sized dopedsemiconductor particles and method of manufacturing same”; and U.S. Pat.No. 5,990,479 to Weiss et al. (Nov. 23, 1999) entitled “Organoluminescent semiconductor nanocrystal probes for biological applicationsand process for making and using such probes.”

Growth of nanowires having various aspect ratios, including nanowireswith controlled diameters, is described in, e.g., Gudiksen et al (2000)“Diameter-selective synthesis of semiconductor nanowires” J. Am. Chem.Soc. 122, 8801-8802; Cui et al. (2001) “Diameter-controlled synthesis ofsingle-crystal silicon nanowires” Appl. Phys. Lett. 78, 2214-2216;Gudiksen et al. (2001) “Synthetic control of the diameter and length ofsingle crystal semiconductor nanowires” J. Phys. Chem. B 105,4062-4064;Morales et al. (1998) “A laser ablation method for the synthesis ofcrystalline semiconductor nanowires” Science 279, 208-211; Duan et al.(2000) “General synthesis of compound semiconductor nanowires” Adv.Mater. 12, 298-302; Cui et al. (2000) “Doping and electrical transportin silicon nanowires” J. Phys. Chem. B 104, 5213-5216; Peng et al.(2000) “Shape control of CdSe nanocrystals” Nature 404, 59-61; Puntes etal. (2001) “Colloidal nanocrystal shape and size control: The case ofcobalt” Science 291, 2115-2117; U.S. Pat. No. 6,306,736 to Alivisatos etal. (Oct. 23, 2001) entitled “Process for forming shaped group III-Vsemiconductor nanocrystals, and product formed using process”; U.S. Pat.No. 6,225,198 to Alivisatos et al. (May 1, 2001) entitled “Process forforming shaped group II-VI semiconductor nanocrystals, and productformed using process”; U.S. Pat. No. 6,036,774 to Lieber et al. (Mar.14, 2000) entitled “Method of producing metal oxide nanorods”; U.S. Pat.No. 5,897,945 to Lieber et al. (Apr. 27, 1999) entitled “Metal oxidenanorods”; U.S. Pat. No. 5,997,832 to Lieber et al. (Dec. 7, 1999)“Preparation of carbide nanorods”; Urbau et al. (2002) “Synthesis ofsingle-crystalline perovskite nanowires composed of barium titanate andstrontium titanate” J. Am. Chem. Soc., 124, 1186; and Yun et al. (2002)“Ferroelectric Properties of Individual Barium Titanate NanowiresInvestigated by Scanned Probe Microscopy” Nanoletters 2, 447.

Growth of branched nanowires (e.g., nanotetrapods, tripods, bipods, andbranched tetrapods) is described in, e.g., Jun et al. (2001) “Controlledsynthesis of multi-armed CdS nanorod architectures using monosurfactantsystem” J. Am. Chem. Soc. 123, 5150-5151; and Manna et al. (2000)“Synthesis of Soluble and Processable Rod-, Arrow-, Teardrop-, andTetrapod-Shaped CdSe Nanocrystals” J. Am. Chem. Soc. 122, 12700-12706.

Synthesis of nanoparticles is described in, e.g., U.S. Pat. No.5,690,807 to Clark Jr. et al. (Nov. 25, 1997) entitled “Method forproducing semiconductor particles”; U.S. Pat. No. 6,136,156 to El-Shall,et al. (Oct. 24, 2000) entitled “Nanoparticles of silicon oxide alloys”;U.S. Pat. No. 6,413,489 to Ying et al. (Jul. 2, 2002) entitled“Synthesis of nanometer-sized particles by reverse micelle mediatedtechniques”; and Liu et al. (2001) “Sol-Gel Synthesis of Free-StandingFerroelectric Lead Zirconate Titanate Nanoparticles” J. Am. Chem. Soc.123, 4344. Synthesis of nanoparticles is also described in the abovecitations for growth of nanocrystals, nanowires, and branched nanowires,where the resulting nanostructures have an aspect ratio less than about1.5.

Synthesis of core-shell nanostructure heterostructures, namelynanocrystal and nanowire (e.g., nanorod) core-shell heterostructures,are described in, e.g., Peng et al. (1997) “Epitaxial growth of highlyluminescent CdSe/CdS core/shell nanocrystals with photostability andelectronic accessibility” J. Am. Chem. Soc. 119, 7019-7029; Dabbousi etal. (1997) “(CdSe)ZnS core-shell quantum dots: Synthesis andcharacterization of a size series of highly luminescent nanocrysallites”J. Phys. Chem. B 101, 9463-9475; Manna et al. (2002) “Epitaxial growthand photochemical annealing of graded CdS/ZnS shells on colloidal CdSenanorods” J. Am. Chem. Soc. 124, 7136-7145; and Cao et al. (2000)“Growth and properties of semiconductor core/shell nanocrystals withInAs cores” J. Am. Chem. Soc. 122, 9692-9702. Similar approaches can beapplied to growth of other core-shell nanostructures.

Growth of nanowire heterostructures in which the different materials aredistributed at different locations along the long axis of the nanowireis described in, e.g., Gudiksen et al. (2002) “Growth of nanowiresuperlattice structures for nanoscale photonics and electronics” Nature415, 617-620; Bjork et al. (2002) “One-dimensional steeplechase forelectrons realized” Nano Letters 2, 86-90; Wu et al. (2002)“Block-by-block growth of single-crystalline Si/SiGe superlatticenanowires” Nano Letters 2, 83-86; and U.S. patent application 60/370,095(Apr. 2, 2002) to Empedocles entitled “Nanowire heterostructures forencoding information.” Similar approaches can be applied to growth ofother heterostructures.

In certain embodiments, the collection or population of nanostructuresis substantially monodisperse in size and/or shape. See e.g., U.S.patent application 20020071952 by Bawendi et al entitled “Preparation ofnanocrystallites.”

Kits

The devices and compositions herein can be packaged as kits. Forexample, any of the devices or compositions of the invention can bepackaged in one or more containers. Similarly, kits can includeinstructional materials that can be used to practice the methods herein,operate the devices herein, use the compositions herein and/or operatethe systems herein. Kits can include other convenient features, such asprotective packaging materials, instructional materials for assembly ofcomponents of the devices or systems, electrical couplings to coupledevices or systems to an electrical input or output, or the like.

Although described in some detail for purposes of understanding, thescope of the claimed invention herein is not limited to the disclosureand is only limited by the claims appended hereto or to any relatedpatent or application, including without limitation any continuation, inwhole or in part, divisional, reissue, reexamination, etc.

EXAMPLES

The following sets forth a series of experiments that demonstrateconstruction of nanocomposite and nanostructure based photovoltaicdevices. It is understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication and scope of the appended claims. Accordingly, the followingexamples are offered to illustrate, but not to limit, the claimedinvention.

Example 1 Nanocomposite Photovoltaic Device

This example describes fabrication of a CdSe nanocrystal-P3HT polymernanocomposite photovoltaic device. CdSe nanorods have been used; CdSenanotetrapods can also be used, as can other nanostructure types and/orcompositions.

Substrate Cleaning

Substrates (e.g., ITO on glass, from Thin Film Devices, Inc.,www.tfdinc.com) are cleaned, e.g., using the following procedure.Substrates are wiped with isopropanol, ultrasonicated in isopropanol,ultrasonicated in 2% Hellmanex™ deionized water, rinsed very thoroughlyunder flowing deionized water, ultrasonicated in deionized water,ultrasonicated in semiconductor grade acetone, and ultrasonicated insemiconductor grade isopropanol. Each sonication is for 15 minutes. Thesubstrates are then oxygen plasma cleaned, at 200 W (1% reflected power)for 10 minutes with oxygen introduced at a pressure of approximately 400mTorr into a vacuum of 80 mTorr.

PEDOT Layer Processing

PEDOT/PSS Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (e.g.,Baytron® P VP AI 4083 from H.C. Starck) is filtered through a 0.2 μmpore size cellulose acetate filter. PEDOT is spin coated onto thesubstrates at 3000 rpm for 60 seconds. The PEDOT layer is then cured bybaking the spincoated substrate on a hotplate at 120° C. for 60 minutesunder atmospheric conditions.

Preparation of P3HT in Chloroform (CHCl3) Solution

The P3HT solution is prepared in a glove-box with an argon atmosphere.Anhydrous chloroform (previously filtered with a 0.2 μm pore size PTFEfilter) is added to the P3HT, such that the concentration of P3HT in theresulting solution is 20 mg/ml. The solution is vortexed for 5 minutes,stirred for approximately 1 hour on a stir plate, and heated at 56° C.for 10 minutes while stirring. The P3HT:chloroform solution is filteredwith a 0.2 μm pore size PTFE filter, and protected from light andoxygen.

Preparation of Nanocrystal Solution

The nanocrystal solution is prepared in the glove-box. CdSe nanocrystalsare dissolved in anhydrous chloroform (previously filtered with a 0.2 μmpore size PTFE filter) at a concentration between 70-80 mg/ml. A smallaliquot of known volume of the CdSe nanocrystal solution is removed fromthe glove box, the nanocrystals are dried under nitrogen flow, and thenanocrystals are weighed to determine the concentration of thenanocrystal solution. (These nanocrystals, which have been exposed tooxygen, are then discarded and are not used to fabricate thephotovoltaic device.) If necessary, additional chloroform is added toadjust the concentration of the nanocrystal solution remaining in theglove box to 70-80 mg/ml.

Preparation of Nanocrystal:P3HT Blend Solution

The CdSe nanocrystal:polymer solution is also prepared in the glove box.The CdSe:CHCl3 solution and the P3HT:CHCl3 solution are combined into amicro-centrifuge tube, such that the ratio of CdSe:P3HT is 90:10 byweight, the concentration of P3HT in the final solution is between 5-7mg/ml, and the concentration of CdSe nanocrystals in the final solutionis between 50-70 mg/ml. For example, if the concentration of CdSe inCHCl3 is 75 mg/ml and the P3HT in CHCl3 is 20.0 mg/ml, 300 μl of CdSenanocrystal solution and 125 μl of P3HT solution are mixed, such thatthe resulting ratio of CdSe:P3HT is 90:10, the resulting concentrationof P3HT is 5.9 mg/ml, and the resulting concentration of CdSe is 52.9mg/ml. The solution is vortexed for 2 minutes and centrifuged for 2minutes at 11,000 rpm in a microcentrifuge.

Spincoating of Nanocrystal:P3HT Blend Solution

The CdSe nanocrystal:P3HT blend is spin coated onto the ITO/PEDOTsubstrates (in the glove box). Typically, 100 μl of solution is used foreach substrate, with a spin speed of 1200 rpm for 40 seconds. Anysolution on the back side of the substrates is removed by wiping withchloroform.

Evaporation of Aluminum Cathodes

The nanocomposite-PEDOT-coated substrates are transferred withoutexposure to oxygen into an evaporator. Aluminum (purity 99.999%) isevaporated onto them at a rate of 5 A/s under a vacuum of less than1×10⁻⁷ torr to a thickness of approximately 200 nm.

Silver Paste

Any nanocomposite and/or PEDOT film on top of the ITO electrode contactpins is removed. Silver paste is applied to establish electricalconnection to the ITO pins. The resulting devices are then characterizedas desired.

Example 2 CdSe—CdTe Nanocrystal Photovoltaic Device

This example describes fabrication of a photovoltaic device comprisingtwo intermixed populations of nanocrystals, CdSe nanocrystals and CdTenanocrystals.

Substrate Cleaning

Substrates (e.g., ITO on glass, from Thin Film Devices, Inc.,www.tfdinc.com) are cleaned, e.g., using the following procedure.Substrates are wiped with isopropanol, ultrasonicated in isopropanol,ultrasonicated in 2% Hellmanex™ deionized water, rinsed very thoroughlyunder flowing deionized water, ultrasonicated in deionized water,ultrasonicated in semiconductor grade acetone, and ultrasonicated insemiconductor grade isopropanol. Each sonication is for 15 minutes. Thesubstrates are then oxygen plasma cleaned, at 200 W (1% reflected power)for 10 minutes with oxygen introduced at a pressure of approximately 400mTorr into a vacuum of 80 mTorr.

PEDOT Layer Processing

PEDOT/PSS Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (e.g.,Baytron® P VP AI 4083 from H.C. Starck) is filtered through a 0.2 μmpore size cellulose acetate filter. PEDOT is spin coated onto thesubstrates at 3000 rpm for 60 seconds. The PEDOT layer is then cured bybaking the spincoated substrate on a hotplate at 120° C. for 60 minutesunder atmospheric conditions.

Preparation of CdSe:CdTe Bicrystal Blend Solution

The CdSe:CdTe bicrystal blend solution is prepared in a glove-box withan argon atmosphere. CdTe nanocrystals are washed by dissolving them intoluene and precipitating them with isopropanol 3 times; CdSenanocrystals are washed by dissolving them in toluene and precipitatingthem with methanol 3 times. For surface treatment, both CdSe and CdTenanocrystals are stirred in a solution of toluene and phenylphosphonicacid (PPA) at 110° C. for 20 hours. (The surface treatment step may notbe necessary and could be omitted, or a different nanocrystal cleaningprocedure, e.g., using pyridine, followed by treatment with PPA oranother ligand may be substituted for this step.) After precipitationwith isopropanol, the nanocrystals are dissolved in toluene, e.g., at aconcentration of 95 mg/ml (for CdTe) and 110 mg/ml (for CdSe),respectively. The CdTe:toluene solution and the CdSe:toluene solutionare combined into a 1.5 ml glass vial, such that the ratio of CdTe:CdSeis 50:50 by weight, and the concentration of nanocrystals in the finalsolution is between about 80-100 mg/ml. For example, if theconcentration of CdTe in toluene is 95 mg/ml and the CdSe in toluene is110 mg/ml, 500 ul of CdTe nanocrystal solution and 432 ul CdSenanocrystal solution are mixed, such that the resulting ratio ofCdTe:CdSe is 50:50 and the resulting concentration of nanocrystals is102 mg/ml. The solution is vortexed for 2 minutes, heated at 56° C. for10 minutes, and ultrasonicated for 15 minutes. After the solution istransferred to a microcentrifuge vial, it is centrifuged for 2 minutesat 11,000 rpm in a microcentrifuge.

Spincoating of CdSe:CdTe Nanocrystal Blend Solution

The CdTe:CdSe solution is spincoated onto the ITO/PEDOT:PSS substrates(in the glove box). Typically, 120 μl of solution is used for eachsubstrate, with a spin speed of 950 rpm for 40 seconds. Any solution onthe back side of the substrates is removed by wiping with chloroform.

Evaporation of Aluminum Cathodes

The nanocrystal-PEDOT-coated substrates are transferred without exposureto oxygen into an evaporator. Aluminum (purity 99.999%) is evaporatedonto them at a rate of 5 A/s under a vacuum of less than 1×10⁻⁷ torr toa thickness of approximately 200 nm.

Silver Paste

Any nanocrystal and/or PEDOT film on top of the ITO electrode contactpins is removed. Silver paste is applied to establish electricalconnection to the ITO pins. The resulting devices are then characterizedas desired.

It will be understood from the foregoing that variations and uses of thepresent invention are encompassed. For example, any of the compositionsof the invention can be used to form devices of the invention. Systemsof the invention can be used to make compositions and/or devices of theinvention. Methods of the invention can be used to make thecompositions, systems or devices herein. Similar variations will beapparent to one of skill.

Furthermore, while the foregoing invention has been described in somedetail for purposes of clarity and understanding, it will be clear toone skilled in the art from a reading of this disclosure that variouschanges in form and detail can be made without departing from the truescope of the invention. For example, all the techniques and apparatusdescribed above can be used in various combinations. All publications,patents, patent applications, and/or other documents cited in thisapplication are incorporated by reference in their entirety for allpurposes to the same extent as if each individual publication, patent,patent application, and/or other document were individually indicated tobe incorporated by reference for all purposes.

1. A photovoltaic device, comprising: a first electrode layer; a secondelectrode layer; and a first photoactive layer disposed between thefirst and second electrode layers, wherein the photoactive layercomprises a first and second population of nanostructures that exhibit atype II band offset energy profile, wherein at least the firstpopulation of nanostructures are not grown from any layer in thephotovoltaic device and include at least one nanostructure which is notin direct contact with any other layer in the device.
 2. Thephotovoltaic device of claim 1, wherein the first population ofnanostructures comprises nanorods.
 3. The photovoltaic device of claim1, wherein the first population of nanostructures comprisesnanotetrapods.
 4. The photovoltaic device of claim 1, wherein the firstand second population of nanostructures comprises nanorods.
 5. Thephotovoltaic device of claim 1, wherein the first and second populationof nanostructures comprises nanotetrapods.
 6. The photovoltaic device ofclaim 1, wherein the first population of nanostructures comprises afirst inorganic material and the second population of nanostructurescomprises a second inorganic material different from the first inorganicmaterial.
 7. The photovoltaic device of claim 1, wherein the first andsecond population of nanostructures comprise at least a portion that iscomprised of a semiconductor selected from Group II-VI, Group III-V orGroup IV semiconductors or alloys thereof.
 8. The photovoltaic device ofclaim 1, wherein the first population of nanostructures comprisesnanorods that comprise one or more of: CdSe, CdTe, InP, InAs, CdS, ZnS,ZnSe, ZnTe, HgTe, GaN, GaP, GaAs, GaSb, InSb, Si, Ge, AlAs, AlSb, PbSe,PbS, or PbTe.
 9. The photovoltaic device of claim 2, wherein thenanorods comprise a core of a first semiconductor material and a shellof a second semiconductor material, which second semiconductor materialis different from the first semiconductor material.
 10. The photovoltaicdevice of claim 9, wherein the core comprises CdSe and the shellcomprises CdTe.
 11. The photovoltaic device of claim 9, wherein the corecomprises InP and the shell comprises GaAs.
 12. The photovoltaic deviceof claim 2, wherein the nanorods have an aspect ratio greater than 5.13. The photovoltaic device of claim 1, wherein the first and secondpopulation of nanostructures are disposed in a conductive polymermatrix.
 14. The photovoltaic device of claim 13, wherein the first andsecond population of nanostructures are coupled to the polymer matrixvia a covalent chemical linkage.
 15. The photovoltaic device of claim13, wherein the first and second population of nanostructures areelectrically coupled to the polymer matrix.
 16. The photovoltaic deviceof claim 1, wherein the first and second population of nanostructuresare disposed in a nonconductive polymer matrix.
 17. The photovoltaicdevice of claim 1, wherein the first and second population ofnanostructures are predominantly positioned closer to the firstelectrode than to the second electrode.
 18. The photovoltaic device ofclaim 1, further comprising a hole or electron blocking layer disposedbetween the photoactive layer and the first or second electrode.
 19. Thephotovoltaic device of claim 1, further comprising a hole blocking layerdisposed between the photoactive layer and the first electrode and anelectron blocking layer disposed between the photoactive layer and thesecond electrode.
 20. The photovoltaic device of claim 1, wherein atleast one of the first and second electrodes are flexible.
 21. Thephotovoltaic device of claim 1, wherein at least one of the first andsecond electrodes comprises a transparent conductive layer.
 22. Thephotovoltaic device of claim 1, wherein the first and second populationof nanostructures comprise nanocrystals having different sizedistributions.
 23. The photovoltaic device of claim 1, furthercomprising: a third electrode layer; a fourth electrode layer; and asecond photoactive layer disposed between the third and fourth electrodelayers.
 24. The photovoltaic device of claim 1, wherein the first andsecond population of nanostructures are randomly oriented in thephotoactive layer.
 25. The photovoltaic device of claim 1, wherein atleast the first population of nanostructures is oriented in thephotoactive layer.